Let f(x) be a positive function and I1 = - 121 2x f(2x(1 - 2x)) dx and I2 = -12 f(x(1 - x)) dx. Then the value of I2I1 is equal to
- 12
- 9
- 6
- 4
Answer
(4) 4
Solution
In I1 put 2x = t dx = dt2; x = - 12 t = -1, x = 1 t = 2
I1 = 12 -12 t f(t(1 - t)) dt ...(i)
Using ab g(t) dt = ab g(a + b - t) dt with a + b = 1 (i.e. t 1 - t, which leaves t(1 - t) unchanged):
I1 = 12 -12 (1 - t) f(t(1 - t)) dt ...(ii)
Adding (i) and (ii):
2I1 = 12 -12 f(t(1 - t)) dt = 12I2
I2 = 4I1 (I1 0, since f > 0 gives I2 > 0)
I2I1 = 4
Answer: (4)