## ZeroAT 2 years ago Calculus - > Integral -> Substitution The question is sin 3x cos 3x dx Solve it by integrating using substitution.

1. Spacelimbus

Do you know what the derivate of $\Large \sin(3x)$ is?

2. Spacelimbus

derivative*

3. geerky42

HINT: Let u be sin(3x).

4. ZeroAT

How do you choose what to let u be? In this case you choose sin(3x) but why?

5. Spacelimbus

Takes some practice, but in case you consider taking u=cos(3x), that would also work. It's mainly about developing the idea of substitution. If you choose u=sin(3x) then: $\Large \frac{du}{dx}=3\cos(3x)$