# Fourier Series Representation of Signals (Proof)

Hi guys,

I was studying the proof below and just can't figure out the the first highlighted step leads to the second and I was wondering if you guys can help me to fill that in. (:

MathematicalPhysicist
Gold Member
It follows from:

cos(a+b)= cos(a)cos(b)-sin(a)sin(b)

It follows from:

cos(a+b)= cos(a)cos(b)-sin(a)sin(b)

Thank you so much! :D

-- it's okay, I for it. Really stupid question. hahaha ---