The number of elements in the set S = \x : x [0, 100] and 0x t2 (x - t) dt = x2 \ is
Answer
16
Solution
Put x - t = u: 0x t2 (x - t) dt = 0x (x - u)2 u du
By parts: = [-(x - u)2 u ]0x - 0x 2(x - u) u du
= x2 - 2 0x (x - u) u du
Again by parts, 0x (x - u) u du = [(x - u) u ]0x + 0x u du = 1 - x, so
0x t2 (x - t) dt = x2 - 2 + 2 x
The given equation becomes x2 - 2 + 2 x = x2, so x = 1 and x = 2n with
0 2n 100 0 n 1002 15.9
so n = 0, 1, 2, , 15.
Number of elements = 16
Answer: 16