Let f: R R be a function defined by f(x)= x (1+x4 )1/4, and g(x)=f(f(f(f(x)))). Then, 18 0 2 5x2g(x) dx is equal to
- 36
- 33
- 39
- 42
Answer
(3) 39
Solution
f(f(x))= x(1+x4)1/4 (1+ x41+x4 )1/4= x(1+2x4)1/4
∵ each further f adds one more x4: g(x)= x(1+4x4)1/4
I=18 0 2 5 x3(1+4x4)1/4dx
Put 1+4x4=t4 16x3dx=4t3dt; x=0 t=1, x4=20 t=3
I= 184 13t2 dt= 92 27-13=39
Answer: (3)