Originally Posted by

**Peleus** Hi all,

Bit of a strange question. When exactly are the double angle formula's needed for the subbing in of angle values?

I.e. Let's say we have an angle of $\displaystyle \frac{\pi}{2}$. I want to sub it into the formula $\displaystyle (3 - \sin 3\theta)$

Can I do it directly? In which cases is subbing it in directly incorrect, and I need to use the double angle formula's? In my learning I've always thought that we needed to simply (which is why the exist) as subbing it in directly leads to the wrong result.

Then I got an integration question where I had to sub in pi/2 into sin5x/5, \Leftarrow is that $\displaystyle \frac{\sin(5x)}{5} $ or $\displaystyle \sin{\frac{5x}{5}} $

and the result was to simply put it in without any simplification.

If you guys could clear up when it is / isn't valid it would be appreciated.

Cheers