Let f : R R be a twice differentiable function such that ( x y)(f(2x + 2y) - f(2x - 2y)) = ( x y)(f(2x + 2y) + f(2x - 2y)), for all x, y R. If f'(0) = 12, then the value of 24f'' ( 5 3 ) is :
- 2
- 3
- -3
- -2
Answer
(3) -3
Solution
Collect f(2x + 2y) and f(2x - 2y):
f(2x + 2y)( x y - x y) = f(2x - 2y)( x y + x y)
f(2x + 2y) (x - y) = f(2x - 2y) (x + y)
f(2(x + y)) (x + y) = f(2(x - y)) (x - y) for all x, y
f(2t) t = k (constant) f(x) = k x2
f'(x) = k2 x2 f'(0) = k2 = 12 k = 1
f(x) = x2, f''(x) = - 14 x2
24f'' ( 5 3 ) = 24 (- 14 ) 5 6 = -6 12 = -3
Answer: (3)