Let f be a twice differentiable non-negative function such that (f(x))2 = 25 + 0x ((f(t))2 + (f'(t))2 )dt. Then the mean of f( e(1)), f( e(2)), , f( e(625)) is equal to .
Answer
1565
Solution
Put x = 0: (f(0))2 = 25 f(0) = 5 (f is non-negative).
Differentiate: 2f(x)f'(x) = (f(x))2 + (f'(x))2
(f(x) - f'(x))2 = 0 f'(x) = f(x)
f(x) = Cex; f(0) = 5 f(x) = 5ex
f( e k) = 5k
Mean = 5(1 + 2 + + 625)625 = 5625 625 6262
= 5 313 = 1565
Answer: 1565