I am trying to figure how to solve the following antiderivative.
$$ \int (5x+3)^7 dx \\ $$ I've seen the step-by-step solution by WolframAlpha however what they are doing in this part:
For the integrand $(5x+3)^7$, substitute $$ u = 5x + 3 \\ du = 5 dx \\ $$
Why are they derivating $u$ ?
Thanks.