How can the change of variables be used to prove these integrals are equal?

[Andrew123]

hi guys just a quick proof i need help with please:

if a = pi / (2-b)

prove the integral from 0 to pi/2 of d(b) / [1 + tan(b)] equals the integral from 0 to pi/2 of

d(a) / [1 + cot(a)]

cheers guys!

 

[CompuChip]

I don't quite see how to get that answer yet, but I'd start doing the change of variables properly.