Hello,

I have an exercise with solution and I don't understand it.

Calculate:

Solution:

Substitution:

will be substituted by and by

Up to here I understand it but now comes the problem.

The book says, that in the integral is still a . Okay. Now they solve the equation and get , so that the hole substitution leads to:

What are the mathematical reasons for the equation ?

I have no idea from where this come from.

They got it from but why is this allowed?

Thanks for help