y=arctan(x)
tany=x
@x=0, tany=0 :. y = 0
@x=1, tany=1 :. y= pi/4
Now we find ∫ [tan (y)] . dy between 0 and pi/4. Answer is ln[sqrt(2)]
The area under the inverse tan curve is equal to the total area of the rectangle (1 x pi/4) minus the area under the tany (w.r.t.y).
So we get pi/4 - ln[sqrt(2)] = pi/4 - [ln(2)]/2
4u, just use IBP.