I've been told that

Question

sin(c+h)=sin(c)xcos(h)+cos(c)xsin(h)

but how?

please explain

Answer 1

Here you will find a proof of that trigonometric identity:

http://library.thinkquest.org/C0110248/trigonometry/formsine.htm

Answer 2

Here is another approach to the proof

http://www.themathpage.com/aTrig/sum-proof.htm

(there seems to be a 'spacing' problem in the html code in the first few lines, but I am sure you know what the equation should read )