If the range of f( ) = 4 + 3 2 4 + 2 , R is [ , ], then the sum of the infinite G.P., whose first term is 64 and the common ratio is , is equal to .
Answer
96
Solution
f( ) = 1 + 2 2 4 + 2
Let 2 = t [0, 1] 4 + 2 = (1 - t)2 + t = t2 - t + 1
t = 0: f = 1 (least value)
t 0: f = 1 + 2t + 1t - 1; t + 1t 2 (equality at t = 1) f 3
[ , ] = [1, 3], common ratio = 13
S = 641 - 13 = 96
Answer: 96