You can
put this solution on YOUR website! Simplify cos(x)cos(y) + sin(x)sin(y)
Such problems as this only require that you have memorized certain
standard double-angle identities.
If you had memorized the identity
cos(A-B) = cos(A)cos(B) + sin(A)sin(B)
The you would have been able to recognize
cos(x)cos(y) + sin(x)sin(y)
as the right side of that identity with x substituted for A, and y
substituted for B, and you'd just make those same substitutions
in the left side of the identity as well and get:
cos(x-y) = cos(x)cos(y) + sin(x)sin(y)
Be sure to memorize the double-angle identities for sine, cosine and tangent
so you'll be able to do such problems on tests.
Edwin
