Question

Stuck on Trig identities! :( Help please~

How do I prove
cos(x+y)cosy + sin(x+y)siny = cosx??

Answer 1

just to understanding the formula :
cosAcosB + sinAsinB = cos(A-B)

Answer 2

Yep I understand

Answer 3

I cant get the answer still though

Answer 4

change A to (x+y) and B to y

Answer 5

I got it! Ha, it was easier than I thought :)

Answer 6

yep, very easy :)

Answer 7

You can have the medal :)

Answer 8

thanks, lol