For t > -1, let t and t be the roots of the equation ((t + 2)1/7 - 1 )x2 + ((t + 2)1/6 - 1 )x + ((t + 2)1/21 - 1 ) = 0 If t -1+ t = a and t -1+ t = b, then 72(a + b)2 is equal to .
Answer
98
Solution
Put h = t + 1; as t -1+, h 0+ and t + 2 = 1 + h.
For small h, (1 + h)1/n - 1 hn, i.e. h 0 (1 + h)1/n - 1h = 1n
Divide the equation by h ( 0) and let h 0+:
17x2 + 16x + 121 = 0
6x2 + 7x + 2 = 0 (multiplying by 42)
(2x + 1)(3x + 2) = 0 x = - 12, - 23
a + b = - 76
72(a + b)2 = 72 4936 = 98
Answer: 98