So I'm trying to get through euler's introduction to the analysis of the infinite so I could eventually read his books on calculus but I'm stuck somewhere and can't seem to figure out how he equates this identity so by expanding I get sin(2y) * cos(z) + cos(2y) * sin(z). I get that the second term of the equation [cos(2y) * sin(z)] gives cos2y - sin2y which gives - sin z if you take the negative out of the factor but I can't seem to figure out how sin(2y) * cos(z) = 2cosy * sin(y + z). I just get 2siny * cosy * cosz and can't seem to do anything else. I'm probably missing something terribly obvious like always happens but I'd really appreciate some help I have nobody I can ask in real life and I don't like to just move on because the book gets more complicated. This is on page 104 if anyone cares to know.