The smallest positive integral value of a, for which all the roots of x4 - ax2 + 9 = 0 are real and distinct, is equal to
- 7
- 3
- 4
- 9
Answer
(1) 7
Solution
Put x2 = t: t2 - at + 9 = 0
Each positive root t gives two real, distinct roots x = t. So all four roots of the given equation are real and distinct ⇔ both roots of the quadratic in t are real, distinct and positive.
∴ D = a2 - 36 > 0, sum of roots = a > 0, product of roots = 9 > 0
⇒ a > 6
∴ Smallest positive integral value of a = 7
Answer: (1)