Let f x = 7 tan 8 ⁡ x + 7 tan 6 ⁡ x - 3 tan 4 ⁡ x - 3 tan 2 ⁡ x   for all x…

Let fx=7tan8x+7tan6x-3tan4x-3tan2x  for all x -π2,π2. Then the correct expression(S) is(are)
  1. 0π4x fxdx=112
  2. 0π4 fxdx=0
  3. 0π4x fxdx=16
  4. 0π4 fxdx=1

Solution

fx=7tan8x+7tan6x-3tan4x-3tan2x
fx=sec2x7tan6x-3tan2x
Now,
0π4sec2x7tan6x-3tan2x dx
=017t6-3t2dt
=7t77-3t33
=17-13=0
Also
0π4xsec2x 7tan6x-3tan2x dx
tanx=t
01tan-1t7t6-3t2 dt
tan-1tt7-t3|01-0111+t2 t7-t3dt
=-0111+t2 t3t4-1dt
=+01t3 1+t2 1-t2dt1+t2=01t3-t5dt
=14-16=112

Asked in: JEE Advanced 2015 (Paper 2)

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