Let a differentiable function f satisfy the equation 036 f ( tx36 )dt = 4 f(x). If y = f(x) is a standard parabola passing through the points (2, 1) and (-4, ), then is equal to .
Answer
64
Solution
Put tx36 = u dt = 36xdu; t: 0 36 u: 0 x
36x 0x f(u) du = 4 f(x)
9 0x f(u) du = x f(x)
Differentiating w.r.t. x:
9f(x) = f(x) + x f'(x)
f'(x)f(x) = 9- 1x
f(x) = cx 9-
Standard parabola y = cx2 9- = 2 = 3
Through (2, 1): 1 = 4c c = 14, i.e. x2 = 4y
Through (-4, ): = 164 = 4
= 43 = 64
Answer: 64