Question

use cosAcosB + sinAsinB to prove cosAcosB - sinAsinB ?

Answer 1

you mean as a "addition angle" formula.

Answer 2

yeah

Answer 3

i believe you are asked to show that
\[\cos(A+B)=\cos(A)\cos(B)-\sin(A)\sin(B)\] or the question makes no sense

Answer 4

in that case note that
\[\cos(A+B)=\cos(A-(-B))\] replace B by -B in the previous formula

Answer 5

get
\[\cos(A-(-B)=\cos(A)\cos(-B)-\sin(A)\sin(-B)\]

Answer 6

and then say
since cosine is even,
\[\cos(-B)=\cos(B)\] and since sine is odd
\[\sin(-B)=-\sin(B)\]

Answer 7

and you get your answer

Answer 8

thanks a lot :D

Answer 9

yw