We looked at problems with integration involving trig functions. That led to reviewing two areas, trig functions of common angles and going over the chain rule. Spent a bit of time on both of those topics which he has seen before but are necessary to solve the current problems.
If you don’t know how to do those things, integrals that involve both aren’t particularly easy to do.
The chain rule is required in many cases.
If you say 12 = 2 x 6, you can’t just leave off the 6. Similarly, you can’t just not do the chain rule when it’s necessary.