Let f(x) be a polynomial of degree 5, and have extrema at x=1 and x=-1. If x 0 ( f(x)x3 )=-5, then f(2)-f(-2) is equal to:
- 0
- 50
- 92
- 112
Answer
(4) 112
Solution
x 0 f(x)x3=-5 (finite, non-zero), f(x) has no terms of degree below 3 and the coefficient of x3 is -5:
f(x)=-5x3+bx4+cx5 (c 0)
f'(x)=-15x2+4bx3+5cx4
f'(1)=0 -15+4b+5c=0
f'(-1)=0 -15-4b+5c=0
b=0, c=3 f(x)=3x5-5x3 (an odd function)
f(2)=96-40=56, f(-2)=-56
f(2)-f(-2)=112
Answer: (4)