If α and β are the zeroes of the polynomial 2 xx 5 8 then the value of α + β is

If α and β are the zeros of the polynomial f(x) = kx2 + 4x + 4 such that α2 + β2 = 24, then find the positive value of k.

  1. \(\frac{1}{3}\)
  2. \(\frac{3}{4}\)
  3. \(\frac{2}{3}\)
  4. \(\frac{4}{3}\)

Answer (Detailed Solution Below)

Option 3 : \(\frac{2}{3}\)

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100 Questions 100 Marks 60 Mins

GIVEN:

α and β are the Roots of the polynomial f(x) = kx2 + 4x + 4.

α2 + β2 = 24

FORMULA USED:

(a + b)2 = a2 + 2ab + b2

Sum of the roots (α + β) = - b / a and Product of roots (αβ) = c / a,  where a = k, b = 4 and c = 4

SOLUTION:

⇒ α + β = - 4 / k

⇒ αβ = 4 / k

⇒ (α + β)2 = α2 + β2 + 2αβ

⇒ ( - 4 / k)2 = 24 + 8 / k

⇒ 16 / k2 = 24 + 8 / k

⇒ 16 / k2 - 8 / k = 24

⇒ (16 - 8k) / k2 = 24

⇒ 16 - 8k = 24 k2

⇒ 24k2 + 8k - 16 = 0

⇒ 3k2 + k - 2 = 0

⇒ 3k2 + 3k - 2k - 2 = 0

⇒ 3k × (k + 1) - 2(k + 1) = 0

⇒ (3k - 2) × (k + 1) = 0

⇒ (3k - 2) = 0

⇒ k = 2 / 3 

⇒ (k + 1) = 0

⇒ k = -1

Positive value of k = 2 / 3

Hence option 3 is the valid answer.

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Solution

Since αandβ are zeroes of the given polynomial, we have Sum of roots (α+β)=−ba=−51=−5 option (b) -5 is correct

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