Let the arithmetic mean of 1a and 1b be 516, a > 2. If is such that a, 4, , b are in A.P., then the equation x2 - ax + 2( - 2b) = 0 has :
- one root in (1, 4) and another in (-2, 0)
- one root in (0, 2) and another in (-4, -2)
- both roots in the interval (-2, 0)
- complex roots of magnitude less than 2
Answer
(1) one root in (1, 4) and another in (-2, 0)
Solution
Since a, 4, , b are in A.P. with common difference 4 - a, = 8 - a and b = 12 - 2a.
1a + 1b = 2 516 = 58
1a + 112 - 2a = 58 12 - aa(12 - 2a) = 58
96 - 8a = 60a - 10a2 5a2 - 34a + 48 = 0
(5a - 24)(a - 2) = 0 a = 245 (since a > 2)
so = 165 and b = 125. The equation becomes
165x2 - 245x + 2 ( 165 - 245 ) = 0
16x2 - 24x - 16 = 0 2x2 - 3x - 2 = 0
(x - 2)(2x + 1) = 0 x = 2, - 12
∴ One root (2) (1, 4) and the other (- 12 ) (-2, 0).
Answer: (1)