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Integrals · Question 35

JEE Main 2025 (Online) 8th April Evening ShiftMCQMedium📈 Topic in 71% papers+4−1
1

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

  1. 12
  2. 9
  3. 6
  4. 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)

JEE Main 2025 (Online) 8th April Evening Shift

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  4. - 4 25
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