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