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Textbook | NCERT |

Class | Class 10 |

Subject | Maths |

Chapter | Chapter 8 |

Chapter Name | Quadratic Equations |

Exercise | 8.6 |

Category | RD Sharma Solutions |

**RD Sharma Solutions for Class 10 Chapter** **8**** Quadratic Equations** Ex 8.6 Download PDF

**Quadratic Equations**Ex 8.6 Download PDF

**Chapter 8: Quadratic Equations Exercise – 8.6**

**Question: 1**

Determine the nature of the roots of the following quadratic equations.

(i) 2x^{2} – 3x + 5 = 0

(ii) 2x^{2} – 6x + 3 = 0

(iii) For what value of k (4 – k)x^{2 }+ (2k + 4)x + (8k + 1) = 0 is a perfect square.

(iv) Find the least positive value of k for which the equation x^{2 }+ kx + 4 = 0 has real roots.

(v) Find the value of k for which the given quadratic equation has real roots and distinct roots.

Kx^{2 }+ 2x + 1 = 0

(vi) Kx^{2} + 6x + 1 = 0

(vii) x^{2 }– kx + 9 = 0

**Solution:**

The given quadratic equation is in the form of ax^{2 }+ bx + c = 0

So a = 2, b = – 3, c = 5

We know, determinant (D) = b^{2} – 4ac = (-3)^{2} – 4(2)(5) = 9 – 40 = – 31 < 0

Since D < 0, the determinant of the equation is negative, so the expression does not having any real roots.

(ii) 2x^{2} – 6x + 3 = 0

The given quadratic equation is in the form of ax^{2 }+ bx + c = 0

So a = 2, b = -6, c = 3

We know, determinant (D) = b^{2} – 4ac = (- 6)^{2} – 4(2)(3) = 36 – 24 = 12 < 0

Since D > 0, the determinant of the equation is positive, so the expression does having any real and distinct roots

(iii) For what value of k (4 – k)x^{2 }+ (2k + 4)x + (8k + 1) = 0 is a perfect square.

The given equation is (4 – k)x^{2 }+ (2k + 4)x + (8k + 1) = 0

Here, a = 4 – k, b = 2k + 4, c = 8k + 1

The discriminate (D) = b^{2 }– 4ac = (2k+4)^{2} – 4(4 – k)(8k + 1)

= (4k^{2 }+ 16 + 16k) – 4(32k + 4 – 8k^{2 }– k)

= 4(k^{2} + 8k^{2 }+ 4k – 31k + 4-4)

= 4(9k^{2 }– 27k)

D = 4(9k^{2 }– 27k)

The given equation is a perfect square D = 0

4(9k^{2 }– 27k) = 0

9k^{2 }– 27k = 0

Taking out common of 3 from both sides and cross multiplying = k^{2 }– 3k = 0

= K (k – 3) = 0

Either k = 0 Or k = 3

The value of k is to be 0 or 3 in order to be a perfect square.

(iv)Find the least positive value of k for which the equation x^{2 }+ kx + 4 = 0 has real roots.

The given equation is x^{2 }+ kx + 4 = 0 has real roots Here, a = 1, b = k, c = 4

The discriminate (D) = b^{2 }– 4ac = 0 = k^{2} – 16 = 0 = k = 4, k = – 4

The least positive value of k = 4 for the given equation to have real roots.

(v) Find the value of k for which the given quadratic equation has real roots and distinct roots.

Kx^{2 }+ 2x + 1 = 0

The given equation is Kx^{2 }+ 2x + 1 = 0

Here, a = k, b = 2, c = 1

The discriminate (D) = b^{2 }– 4ac = 0

= 4 – 4k = 0

= 4k = 4

K = 1

The value of k = 1 for which the quadratic equation is having real and equal roots.

(vi) Kx^{2} + 6x + 1 = 0

The given equation is Kx^{2 }+ 6x + 1 = 0

Here, a = k, b = 6, c = 1

The discriminate (D) = b^{2 }– 4ac = 0

= 36 – 4k = 0

= 4k = 36

K = 9

The value of k = 9 for which the quadratic equation is having real and equal roots.

(vii) x^{2 }– kx + 9 = 0

The given equation is x^{2 }– kx + 9 = 0

Here, a = 1, b = – k, c = 9

Given that the equation is having real and distinct roots.

Hence, the discriminate (D) = b^{2 }– 4ac = 0

= k^{2} – 4(1)(9) = 0

= k^{2 }– 36 = 0

= K = – 6 and k = 6

The value of k lies between -6 and 6 respectively to have the real and distinct roots.

**Question: 2**

Find the value of k (i) Kx^{2 }+ 4x + 1 = 0.

(ii)

(iii) 3x^{2 }– 5x + 2k = 0

(iv) 4x^{2 }+ kx + 9 = 0

(v) 2kx^{2 }– 40x + 25 = 0

(vi) 9x^{2 }– 24x + k = 0

(vii) 4x^{2}– 3kx + 1 = 0

(viii) x^{2 }– 2(5 + 2k)x + 3(7 + 10k) = 0

(ix) (3k +1)x^{2}+ 2(k +1)x + k = 0

(x) Kx^{2 }+ kx + 1 = – 4x^{2 }– x

(xi) (k + 1)x^{2 }+ 2(k + 3)x + k + 8 = 0

(xii) x^{2 }– 2kx + 7k – 12 = 0

(xiii) (k + 1)x^{2 }– 2(3k + 1)x + 8k + 1 = 0

(xiv) 5x^{2 }– 4x + 2 + k(4x^{2 }– 2x + 1) = 0

(xv) (4 – k)x^{2 }+ (2k + 4)x + (8k + 1) = 0

(xvi) (2k + 1)x^{2 }+ 2(k + 3)x + (k +5 ) = 0

(xvii) 4x^{2 }– 2(k + 1)x + (k + 4) = 0

**Solution:**

The given equation Kx^{2 }+ 4x + 1 = 0 is in the form of ax^{2 }+ bx + c = 0

Where a = k, b = 4, c = 1

Given that, the equation has real and equal roots D = b^{2 }– 4ac = 0

= 4^{2 }– 4(k)(1) = 0

= 16 – 4k = 0

= k = 4

The value of k is 4

(ii)

The given equationis in the form of ax^{2 }+ bx + c = 0 where a= k, b = – 2√5, c = 4.

Given that, the equation has real and equal roots D = b^{2}– 4ac = 0

20 – 16k = 0

K = 5 /4

The value of k is k = 5/4

(iii) 3x^{2 }– 5x + 2k = 0

The given equation 3x^{2 }– 5x + 2k = 0 is in the form of ax^{2 }+ bx + c = 0 where a = 3, b = – 5, c = 2k

Given that, the equation has real and equal roots D = b^{2 }– 4ac = 0

= (- 5)^{2 }– 4(3)(2k) = 0

= 25 – 24k = 0

K = 25/24

The value of the k is k = 25/24

(iv) 4x^{2 }+ kx + 9 = 0

The given equation 4x^{2 }+ kx + 9 = 0 is in the form of ax^{2 }+ bx + c = 0 where a = 4, b = k, c = 9

Given that, the equation has real and equal roots D = b^{2 }– 4ac = 0

= k^{2 }– 4(4)(9) = 0

= k^{2 }– 144 = 0

= k = 12

The value of k is 12

(v) 2kx^{2 }– 40x + 25 = 0

The given equation 2kx^{2 }– 40x + 25 = 0 is in the form of ax^{2 }+ bx + c = 0 where a = 2k, b = – 40, c = 25

Given that, the equation has real and equal roots D = b^{2 }– 4ac = 0

(-40)^{2 }– 4(2k)(25) = 0

1600 – 200k = 0

k = 8

The value of k is 8

(vi) 9x^{2 }– 24x + k = 0

The given equation 9x^{2 }– 24x + k = 0 is in the form of ax^{2 }+ bx + c = 0 where a = 9, b = – 24, c = k

Given that, the equation has real and equal roots D = b^{2 }– 4ac = 0

( – 24)^{2 }– 4(9)(k) = 0

576 – 36k = 0

k = 16

The value of k is 16

(vii) 4x^{2}– 3kx + 1 = 0

The given equation 4x^{2 }– 3kx + 1 = 0 is in the form of ax^{2 }+ bx + c = 0 where a = 4, b = – 3k, c = 1

Given that, the equation has real and equal roots D = b^{2 }– 4ac = 0

= (-3k)^{2 }– 4(4)(1) = 0

= 9k^{2 }– 16 = 0

K = 4/3

The value of k is 4/3

(viii) x^{2 }– 2(5 + 2k)x + 3(7 + 10k) = 0

The given equation X^{2 }– 2(5 + 2k)x + 3(7 + 10k) = 0 is in the form of ax^{2 }+ bx + c = 0 where a = 1, b = +2(52k), c = 3(7 + 10k)

Given that, the nature of the roots of the equation are real and equal roots D = b^{2 }– 4ac = 0

= (+2(52k))^{2 }– 4(1)(3(7 + 10k)) = 0

= 4(5 + 2k)^{2 }– 12(7 + 10k) = 0

= 25 + 4k^{2 }+ 20k – 21 – 30k = 0

= 4k^{2 }– 10k + 4 = 0

Simplifying the above equation.

We get, = 2k^{2 }– 5k + 2 = 0

= 2k^{2 }– 4k – k + 2 = 0

= 2k(k – 2) – 1(k – 2) = 0

= (k – 2)(2k – 1) = 0, K = 2 and k = 1/2 The value of k can either be 2 or 1/2

(ix) (3k +1)x^{2}+ 2(k +1)x + k = 0

The given equation (3k + 1)x^{2 }+ 2(k + 1)x + k = 0 is in the form of ax^{2 }+ bx + c = 0 where a = 3k + 1, b = +2(k + 1), c = (k)

Given that, the nature of the roots of the equation are real and equal roots D = b^{2 }– 4ac = 0

= [2(k + 1)]^{2 }– 4(3k + 1)(k) = 0

= (k + 1)^{2 }– k(3k + 1) = 0

= -2k^{2 }+ k + 1 = 0

This equation can also be written as 2k^{2 }– k – 1 = 0

The value of k can be obtained by k

The value of k are 1 and (-1)/2 respectively.

(x) Kx^{2 }+ kx + 1 = -4x^{2 }– x

Bringing all the x components on one side we get, x^{2}(4 + k) + x(k + 1) + 1 = 0

The given equation Kx^{2 }+ kx + 1 = -4x^{2 }– x is in the form of ax^{2 }+ bx + c = 0 where a = 4 + k,b = +k + 1, c = 1 Given that, the nature of the roots of the equation are real and equal roots D= b^{2}– 4ac = 0

= (k+1)^{2}– 4(4 + k)(1) = 0

= k^{2}– 2k – 10 = 0

The equation is also in the form ax^{2 }+ bx + c = 0

The value of k is obtained by a = 1, b = -2, c = – 15

Putting the respective values in the above formula we will obtain the value of k

The value of k are 5 and -3 for different given quadratic equation.

(xi) (k + 1)x^{2 }+ 2(k + 3)x + k + 8 = 0

The given equation (k + 1) x^{2 }+ 2(k + 3)x + k + 8 = 0 is in the form of ax^{2 }+ bx + c = 0 where a = k + 1,b = 2(k + 3), c = k + 8

Given the nature of the roots of the equation are real and equal. D = b^{2 }– 4ac = 0

= [2(k + 30]^{2 }– 4(k + 1)(k + 8) = 0

= 4(k + 3)^{2 }– 4(k + 1)(k + 8) = 0

Taking out 4 as common from the LHS of the equation and dividing the same on the RHS = (k + 3)^{2}-(k + 1)(k + 8) = 0

= k^{2 }+ 9 + 6k – (k^{2 }+ 9k + 18) = 0

Cancelling out the like terms on the LHS side = 9 + 6k – 9k – 8 = 0

= – 3k + 1 = 0

= 3k = 1

K = 1/3

The value of k of the given equation is k =1/3

(xii) x^{2 }– 2kx + 7k – 12 = 0

The given equation is x^{2 }– 2kx + 7k – 12 = 0

The given equation is in the form of ax^{2 }+ bx + c = 0 where a = 1, b = – 2k, c = 7k – 12

Given the nature of the roots of the equation are real and equal. D = b^{2 }– 4ac = 0

= (2k)^{2 }– 4(1)(7k – 12) = 0

= 4k^{2 }– 28k + 48 = 0

= k^{2 }– 7k + 12 = 0

The value of k can be obtained by

Here a = 1, b = – 7k, c = 12

By calculating the value of k is

The value of k for the given equation is 4 and 3 respectively.

(xiii) (k + 1)x^{2 }– 2(3k + 1)x + 8k + 1 = 0

The given equation is (k + 1)x^{2 }– 2(3k + 1)x + 8k + 1 = 0

The given equation is in the form of ax^{2 }+ bx + c = 0 where a = k + 1, b = – 2(k + 1), c = 8k + 1

Given the nature of the roots of the equation are real and equal. D = b^{2}– 4ac = 0

= (-2(k + 1))^{2 }– 4(k + 1)(8k + 1) = 0

= 4(3k + 1)^{2 }– 4(k + 1)(8k + 1) = 0

Taking out 4 as common from the LHS of the equation and dividing the same on the

RHS = (3k + 1)^{2 }– (k + 1)(8k + 1) = 0

= 9k^{2 }+ 6k + 1 – (8k^{2 }+ 9k + 1) = 0

= 9k^{2 }+ 6k + 1 – 8k^{2 }– 9k – 1 = 0

= k^{2 }– 3k = 0

= k(k – 3) = 0

Either k = 0 Or, k – 3 = 0 = k = 3

The value of k for the given equation is 0 and 3 respectively.

(xiv) 5x^{2 }– 4x + 2 + k(4x^{2 }– 2x + 1) = 0

The given equation 5x^{2 }– 4x + 2 + k(4x^{2 }– 2x + 1) = 0 can be written as x^{2}(5 + 4k) – x(4 + 2k) + 2 – k = 0

The given equation is in the form of ax^{2 }+ bx + c = 0 where a = 5 + 4k, b = -(4 + 2k), c = 2 – k

Given the nature of the roots of the equation are real and equal.

D = b^{2 }– 4ac = 0

= [-(4 + 2k)]^{2 }– 4(5 + 4k)(2 – k) = 0

= 16 + 4k^{2 }+ 16 – 4(10 – 5k + 8k – 4k^{2}] = 0

= 16 + 4k^{2 }+ 16 – 40 + 20k – 32k + 16k^{2} = 0

= 20k^{2 }– 4k – 24 = 0

Taking out 4 as common from the LHS of the equation and dividing the same on the RHS = 5k^{2 }– k – 6 = 0 The value of k can be obtained by equation

The value of k for the given equation are k = 6/5 and−1 respectively.

(xv) (4 – k)x^{2 }+ (2k + 4)x + (8k + 1) = 0

The given equation is (4 – k)x^{2 }+ (2k + 4)x + (8k + 1) = 0

The given equation is in the form of ax^{2 }+ bx + c = 0 where a = 4 – k, b = (2k + 4), c = 8k + 1

Given the nature of the roots of the equation are real and equal.

D = b^{2 }– 4ac = 0

= (2k + 4)^{2 }– 4(4 – k)(8k + 1) = 0

= 4k^{2 }+ 16k + 16 – 4(-8k^{2 }+ 32k + 4 – k) = 0

= 4k^{2 }+ 16k + 16 + 32k^{2 }– 124k – 16 = 0

Cancelling out the like and opposite terms.

We get, = 36k^{2 }– 108k = 0

Taking out 4 as common from the LHS of the equation and dividing the same on the RHS = 9k^{2 }– 27k = 0

= 9k(k -3 ) = 0

Either 9k = 0 K = 0 Or, k – 3 = 0 K = 3

The value of k for the given equation is 0 and 3 respectively.

(xvi) (2k + 1)x^{2 }+ 2(k + 3)x + (k +5 )= 0

The given equation is (2k + 1)x^{2 }+ 2(k + 3)x + (k + 5) = 0

The given equation is in the form of ax^{2 }+ bx + c = 0 where a = 2k + 1, b = 2(k + 3), c = k + 5

Given the nature of the roots of the equation are real and equal.

D = b^{2 }– 4ac = 0

= [2(k + 3)]^{2 }– 4(2k + 1)(k + 5) = 0

Taking out 4 as common from the LHS of the equation and dividing the same on the

RHS = [(k + 3)]^{2 }– (2k + 1)(k + 5) = 0

= K^{2 }+ 9 + 6k – (2k^{2 }+ 11k + 5) = 0

= – k^{2 }– 5k + 4 = 0

= k^{2 }+ 5k – 4 = 0

The value of k can be obtained by k = 6/5 and − 1 respectively.

Here a = 1, b = 5, c = – 4

The value of k for the given equation is

(xvii) 4x^{2 }– 2(k + 1)x + (k + 4) = 0

The given equation is 4x^{2 }– 2(k + 1)x + (k + 4) = 0

The given equation is in the form of ax^{2 }+ bx + c = 0 where a = 4, b = -2(k + 1), c = k + 4

Given the nature of the roots of the equation are real and equal.

D = b^{2 }– 4ac = 0

= [-2(k + 1)]^{2 }– 4(4)(k + 4) = 0

Taking out 4 as common from the LHS of the equation and dividing the same on the

RHS = (k + 1)^{2 }– 4(k + 4) = 0

= k^{2 }+ 1 + 2k – 4k – 16 = 0

= k^{2 }– 2k – 15 = 0

The value of k can be obtained by k = 6/5 and − 1 respectively.

Here a = 1, b = -2, c = -15

The value of k for the given equation is

**Question: 3**

In the following, determine the set of values of k for which the given quadratic equation has real roots:

(i) 2x^{2 }+ 3x + k = 0

(ii) 2x^{2 }+ kx + 3 = 0

(iii) 2x^{2 }– 5x – k = 0

(iv) Kx^{2 }+ 6x + 1= 0

(v) x^{2 }– kx + 9 = 0

**Solution:**

(i) 2x^{2 }+ 3x + k = 0

The given equation is 2x^{2 }+ 3x +k = 0

The given quadratic equation has equal and real roots D = b^{2}– 4ac = 0

The given equation is in the form of ax^{2 }+ bx + c = 0 so, a = 2, b = 3, c = k = 9 – 4(2)(k) = 0

= 9 – 8k = 0

= k ≤ 98

The value of k does not exceed k ≤ 98 to have a real root.

(ii) 2x^{2 }+ kx + 3 = 0

The given equation is 2x^{2 }+ kx + 3 = 0

The given quadratic equation has equal and real roots D = b^{2 }– 4ac = 0

The given equation is in the form of ax^{2 }+ bx + c = 0 so, a = 2, b = k, c = 3 = k^{2 }– 4(2)(3) = 0

= k^{2 }– 24 = 0

The value of k should not exceedin order to obtain real roots.

**(iii) 2x ^{2 }– 5x – k = 0**

The given equation is 2x^{2 }– 5x – k = 0

The given quadratic equation has equal and real roots D = b^{2 }– 4ac = 0

The given equation is in the form of ax^{2 }+ bx + c = 0 so, a = 2, b = – 5, c = – k = 25 – 4(2)(- k) = 0

= 25 – 8k = 0

= k ≤ 25/8

The value of k should not exceed k ≤ 25/8 **(iv) Kx ^{2 }+ 6x + 1= 0**

The given equation is Kx^{2 }+ 6x + 1 = 0

The given quadratic equation has equal and real roots D = b^{2 }– 4ac = 0

The given equation is in the form of ax^{2 }+ bx + c = 0 so, a = k, b = 6, c = 1 = 36 – 4(k)(1) = 0

= 36 – 4k = 0

= k = 9

The value of k for the given equation is k = 9

**(v) x ^{2 }– kx + 9 = 0**

The given equation is X^{2 }– kx + 9 = 0

The given quadratic equation has equal and real roots D = b^{2}– 4ac = 0

The given equation is in the form of ax^{2 }+ bx + c = 0 so, a = 1, b = -k, c = 9 = k^{2 }– 4(1)(-9) = 0

= k^{2 }– 36 = 0

= k^{2 }= 36 k ≥ √36 K = 6 and k = – 6

The value of k should in between K = 6 and k = – 6 in order to maintain real roots.

**Question: 4**

**Determine the nature of the roots of the following quadratic equations.**

(i) 2x^{2} – 3x + 5 = 0

(ii) 2x^{2} – 6x + 3 = 0

(iii) For what value of k (4 – k)x^{2 }+ (2k + 4)x + (8k + 1) = 0 is a perfect square

(iv) Find the least positive value of k for which the equation x^{2 }+ kx + 4 = 0 has real roots.

(v) Find the value of k for which the given quadratic equation has real roots and distinct roots.

Kx^{2 }+ 2x + 1 = 0

(vi) Kx^{2} + 6x + 1 = 0

(vii) x^{2 }– kx + 9 = 0

**Solution:**

(i) 2x^{2} – 3x + 5 = 0

The given quadratic equation is in the form of ax^{2 }+ bx + c = 0

So a = 2, b = -3, c = 5

We know, determinant (D) = b^{2} – 4ac = (-3)^{2} – 4(2)(5)

= 9 – 40 = – 31 < 0

Since D < 0, the determinant of the equation is negative, so the expression does not having any real roots.

(ii) 2x^{2} – 6x + 3 = 0

The given quadratic equation is in the form of ax^{2 }+ bx + c = 0

So a = 2, b = -6, c = 3

We know, determinant (D) = b^{2} – 4ac = (-6)^{2} – 4(2)(3) = 36 – 24 = 12 < 0

Since D > 0, the determinant of the equation is positive, so the expression does having any real and distinct roots.

(iii) For what value of k (4 – k)x^{2 }+ (2k + 4)x + (8k + 1) = 0 is a perfect square

The given equation is (4 – k)x^{2 }+ (2k + 4)x + (8k + 1) = 0

Here, a = 4 – k, b = 2k + 4, c = 8k + 1

The discriminate (D) = b^{2 – }4ac

= (2k + 4)^{2} – 4(4 – k)(8k + 1)

= (4k^{2 }+ 16 + 16k) – 4(32k + 4 – 8k^{2 }– k)

= 4(k^{2 }+ 8k^{2 }+ 4k – 31k + 4 – 4)

= 4(9k^{2 }– 27k)

= 4(9k^{2 }– 27k)

The given equation is a perfect square D = 0

4(9k^{2 }– 27k) = 0

9k^{2 }– 27k = 0

Taking out common of 3 from both sides and cross multiplying K^{2 }– 3k = 0 K (k – 3) = 0

Either k = 0 Or k = 3

The value of k is to be 0 or 3 in order to be a perfect square.

(iv) Find the least positive value of k for which the equation x^{2 }+ kx + 4 = 0 has real roots.

The given equation is x^{2 }+ kx + 4 = 0 has real roots Here, a = 1, b = k, c = 4

The discriminate (D) = b^{2 – }4ac = 0

= k^{2} – 16 = 0

= k = 4, k = – 4

The least positive value of k = 4 for the given equation to have real roots.

(v) Find the value of k for which the given quadratic equation has real roots and distinct roots.

Kx^{2 }+ 2x + 1 = 0

The given equation is Kx^{2 }+ 2x + 1 = 0

Here, a = k, b = 2, c = 1

The discriminate (D) = b^{2 – }4ac = 0

= 4 – 4k = 0

= 4k = 4

K = 1

The value of k = 1 for which the quadratic equation is having real and equal roots.

(vi) Kx^{2} + 6x + 1 = 0

The given equation is Kx^{2 }+ 6x + 1 = 0

Here, a = k, b = 6, c = 1

The discriminate (D) = b^{2 – }4ac = 0

= 36 – 4k = 0

= 4k = 36

= K = 9

The value of k = 9 for which the quadratic equation is having real and equal roots.

(vii) x^{2 }– kx + 9 = 0

The given equation is X^{2 }– kx + 9 = 0

Here, a = 1, b = -k, c = 9

Given that the equation is having real and distinct roots. Hence, the discriminate (D) = b^{2 – }4ac = 0

= k^{2} – 4(1)(9) = 0

= k^{2 }– 36 = 0

= k = – 6 and k = 6

The value of k lies between -6 and 6 respectively to have the real and distinct roots.

**Question: 5**

Find the values of k for which the given quadratic equation has real and distinct roots.

(i) Kx^{2 }+ 2x + 1 = 0

(ii) Kx^{2 }+ 6x + 1 = 0

**Solution:**

(i) Kx^{2 }+ 2x + 1 = 0

The given equation is Kx^{2 }+ 2x + 1 = 0

The given equation is in the form of ax^{2 }+ bx + c = 0 so, a = k, b = 2, c = 1 D = b^{2 }– 4ac = 0

= 4 – 4(1)(k) = 0

= 4k = 4

k = 1

The value of k for the given equation is k = 1

(ii) Kx^{2 }+ 6x + 1 = 0

The given equation is Kx^{2 }+ 6x + 1 = 0

The given equation is in the form of ax^{2 }+ bx + c = 0 so, a = k, b = 6, c = 1

D = b^{2 }– 4ac = 0

= 36 – 4(1)(k) = 0

= 4k = 36

= k = 9

The value of k for the given equation is k = 9

**Question: 6**

**For what value of k, (4 – k)x ^{2 }+ (2k + 4)x + (8k + 1) = 0, is a perfect square.**

**Solution:**

The given equation is (4 – k)x^{2 }+ (2k + 4)x + (8k + 1) = 0

The given equation is in the form of ax^{2 }+ bx + c = 0 so, a = 4 – k, b = 2k + 4, c = 8k + 1 D = b^{2 }– 4ac

= (2k + 4)^{2 }– 4(4 – k)(8k + 1)

= 4k^{2 }+ 16 + 4k – 4(32 + 4 – 8k^{2 }– k)

= 4(k^{2 }+ 4 + k – 32 – 4 + 8k^{2 }+ k)

=4(9k^{2 }– 27k)

Since the given equation is a perfect square Therefore D = 0 = 4(9k^{2 }– 27k) = 0

= (9k^{2 }– 27k) = 0

= 3k(k – 3) = 0

Therefore 3k = 0

K = 0 Or, k-3 = 0 K = 3 The value of k should be 0 or 3 to be perfect square.

**Question: 7**

**If the roots of the equation (b – c)x ^{2} + (c – a)x + (a – b) = 0 are equal , then prove that 2b = a + c.**

**Solution:**

The given equation is (b – c)x^{2} + (c – a)x + (a – b) = 0.

The given equation is the form of ax^{2 }+ bx + c = 0.

So, a = (b – c), b = (c – a), c = (a – b)

According to question the equation is having real and equal roots.

Hence discriminant (D) = b^{2 }– 2ac = 0

= (c – a)^{2 }– 4(b – c)(a – b) = 0

= c^{2 }+ a^{2 }– 2ac – 4(ab – b^{2 }– ac + cb) = 0

= c^{2 }+ a^{2 }– 2ac – 4ab + 4b^{2 }+ 4ac – 4cb = 0

= c^{2 }+ a^{2 }+ 2ac – 4ab + 4b^{2 }– 4cb = 0

= (a + c)^{2 }– 4ab + 4b^{2 }– 4cb = 0

= (c + a – 2b)^{2} = 0

= (c + a – 2b) = 0

= c + a = 2b

Hence it is proved that c + a = 2b.

**Question: 8**

**If the roots of the equation (a ^{2 }+ b^{2}) x^{2 }– 2(ac + bd)x + (c^{2 }+ d^{2}) = 0 are equal. Prove that a ÷ b = c ÷ d.**

**Solution:**

The given equation is (a^{2 }+ b^{2})x^{2} – 2(ac + bd)x + (c^{2 }+ d^{2}) = 0.

The equation is in the form of ax^{2 }+ bx = c = 0

Hence, a = (a^{2 }+ b^{2}), b = – 2(ac + bd), c = (c^{2 }+ d^{2}).

The given equation is having real and equal roots.

Discriminant (D) = b^{2 }– 4ac = 0

= [-2(ac + bd)]^{2} – 4 (a^{2 }+b^{2})(c^{2 }+ d^{2}) = 0

= (ac + bd)^{2} – (a^{2 }+ b^{2})(c^{2 }+ d^{2}) = 0

= a^{2}c^{2} + b^{2}d^{2 }+ 2abcd – (a^{2}c^{2} + a^{2}d^{2} + b^{2}c^{2 }+ b^{2}d^{2}) = 0

Cancelling out the equal and opposite terms.

We get, = 2abcd – a^{2}d^{2} – b^{2}c^{2 }= 0

= abcd + abcd – a^{2}d^{2} – b^{2}c^{2 }= 0

= ad(bc – ad) + bc(ad – bc) = 0

= ad(bc – ad) – bc(bc – ad) = 0

= (ad – bc)(bc – ad) = 0

= ad – bc = 0

= (a ÷ b) = (c ÷ d)

Hence, it is proved.

**Question: 9**

If the roots of the equation ax^{2 }+ 2bx + c = 0 andare simultaneously real, then prove that b^{2 }– ac = 0.

**Solution:**

The given equations are ax^{2 }+ 2bx +c = 0 and

These two equations are of the form ax^{2 }+ bx + c = 0.

Given that the roots of the two equations are real.

Hence, D = 0 that is b^{2 }– 4ac = 0

Let us assume that ax^{2 }+ 2bx + c = 0 be equation (i) andbe (ii) From equation (i) b^{2 }– 4ac = 0

= 4 b^{2 }– 4ac = 0 …. (iii)

From equation (ii) b^{2 }– 4ac = 0

Given, that the roots of equation (i) and (ii) are simultaneously real and hence equation (iii) = equation (iv).

= 4b^{2 }– 4ac = 4ac – 4

b^{2} = 8ac = 8b^{2} = b^{2 }– ac = 0.

Hence it is proved that b^{2 }– ac = 0.

**Question: 10**

**If p, q are the real roots and p = q. Then show that the roots of the equation (p-q)x ^{2 }+ 5(p + q)x – 2(p – q) = 0 are real and equal.**

**Solution:**

The given equation is (p – q)x^{2} + 5(p + q)x – 2(p – q) = 0

Given, p , q are real and p ? q.

Then, Discriminant (D) = b^{2}– ac = [5(p + q)]^{2} – 4(p – q)(-2(p – q)) = 25(p + q)^{2} + (p – q)^{2}

We know that the square of any integer is always positive that is, greater than zero.

Hence, (D) = b^{2 }–ac = 0 As given, p, q are real and p = q.

Therefore, = 25(p + q)^{2} + (p – q)^{2 }? 0 = D = 0

Therefore, the roots of this equation are real and unequal.

**Question: 11**

**If the roots of the equation (c ^{2 }– ab)x^{2 }– 2(a^{2 }– bc)x + b^{2 }– ac = 0 are equal , then prove that either a = 0 or a³ + b³ + c³ = 3abc .**

**Solution:**

The given equation is (c^{2 }– ab)x^{2 }– 2(a^{2 }– bc)x + b^{2 }– ac = 0

This equation is in the form of ax^{2 }+ bx + c = 0

So, a = (c^{2 }– ab), b = -2(a^{2 }– bc), c = b^{2 }– ac.

According to the question, the roots of the given question are equal.

Hence, D= 0, b^{2 }– 4ac = 0

= [-2(a^{2 }– bc)]^{2} – 4(c^{2 }– ab)( b^{2 }– ac) = 0

= 4(a^{2 }– bc)^{2} – 4(c^{2 }– ab)( b^{2 }– ac) = 0

= 4a(a³ + b³ + c³ – 3abc) = 0

Either 4a =0 therefore, a = 0 Or, (a³ + b³ + c³ – 3abc) = 0

= (a³ + b³ + c³) = 3abc Hence its is proved.

**Question: 12**

Show that the equation 2(a^{2 }+ b^{2})x^{2 }+ 2(a + b)x + 1 = 0 has no real roots , when a = b.

**Solution:**

The given equation is 2(a^{2 }+ b^{2})x^{2 }+ 2(a + b)x – 1 = 0

This equation is in the form of ax^{2 }+ bx + c = 0

Here, a = 2(a^{2 }+ b^{2}), b = 2(a + b), c = +1.

Given, a = b The discriminant (D) = b^{2 }– 4ac = [2(a + b)]^{2} – 4(2(a^{2 }+ b^{2}))(1)

= 4(a + b)^{2} – 8(a^{2 }+ b^{2})

= 4(a^{2 }+ b^{2 }+ 2ab) – 8a^{2 }– 8b^{2}

= +2ab – 4a^{2 }– 4b^{2}

According to the question a = b, as the discriminant D has negative squares so the value of D will be less than zero. Hence, D = 0, when a = b.

**Question: 13**

**Prove that both of the roots of the equation (x – a)(x – b) +(x – c)(x – b) + (x – c)(x – a) = 0 are real but they are equal only when a = b = c.**

**Solution:**

The given equation is (x – a)(x – b) + (x – c)(x – b) + (x – c)(x – a) = 0

By solving the equation, we get it as, 3x^{2 }– 2x(a + b + c) + (ab + bc + ca) = 0

This equation is in the form of ax^{2 }+ bx + c = 0

Here, a = 3, b = 2(a + b + c), c = (ab + bc + ca)

The discriminate (D) = b^{2 }– 4ac

= [-2(a + b + c)]^{2 }– 4(3)(ab + bc + ca)

= 4(a + b +c )^{2 }-12(ab + bc + ca)

= 4[(a + b + c)^{2 }– 3(ab + bc + ca)]

= 4[a^{2 }+ b^{2 }+ c^{2 }– ab – bc – ca]

= 2[2a^{2 }+ 2b^{2 }+ 2c^{2 }– 2ab – 2bc – 2ca]

= 2[(a – b)^{2 }+ (b – c)^{2 }+ (c – a)^{2}]

Here clearly D = 0, if D = 0 then, [(a – b)^{2 }+ (b – c)^{2 }+ (c – a)^{2}] = 0

a –b = 0 b – c = 0 c – a = 0

Hence, a = b = c = 0 Hence, it is proved.

**Question: 14**

**If a, b, c are real numbers such that ac = 0, then, show that at least one of the equations ax ^{2 }+ bx + c = 0 and – ax^{2 }+ bx + c = 0 has real roots.**

**Solution:**

The given equation are ax^{2 }+ bx + c = 0 … (i)

And – ax^{2 }+ bx + c = 0 … (ii)

Given, equations are in the form of ax^{2}+ bx + c = 0 also given that a, b, c are real numbers and ac = 0.

The Discriminant (D) = b^{2 }– 4ac For equation (i) = b^{2 }– 4ac … (iii)

For equation (ii) = b^{2 }– 4(-a)(c) = b^{2 }+ 4ac … (iv)

As a, b, c are real and given that ac = 0

Hence b^{2 }– 4ac = 0 and b^{2}+ 4ac = 0 Therefore, D = 0 Hence proved.

**Question: 15**

If the equation (1 + m^{2})x + 2mcx + (c^{2 }– a^{2}) = 0 has real and equal roots , prove that c^{2 }= a^{2}(1 + m^{2}).

**Solution:**

The given equation is (1 + m^{2})x^{2 }+ 2mcx + (c^{2 }– a^{2}) = 0

The above equation is in the form of ax^{2 }+ bx + c = 0.

Here a = (1 + m^{2}), b = 2mc, c = +(c^{2 }– a^{2})

Given, that the nature of the roots of this equation is equal and hence D = 0, b^{2 }– 4ac = 0

= (2mc)^{2} – 4(1 + m^{2})(c^{2 }– a^{2}) = 0

= 4m^{2}c^{2 }– 4(c^{2 }+ m^{2}c^{2 }– a^{2} – a^{2}m^{2}) = 0

= 4(m^{2}c^{2 }– c^{2 }+ m^{2}c^{2 }+ a^{2} + a^{2}m^{2}) = 0

= m^{2}c^{2 }– c^{2 }+ m^{2}c^{2 }+ a^{2} + a^{2}m^{2 }= 0

Now cancelling out the equal and opposite terms, = a^{2} + a^{2}m^{2 }– c^{2 }= 0

= a^{2} (1 + m^{2}) – c^{2} = 0

Therefore, c^{2 }= a^{2} (1 + m^{2}) Hence it is proved that as D = 0, then the roots are equal of c^{2 }= a^{2} (1+ m^{2}).

**All Chapter RD Sharma Solutions For Class10 Maths**

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