SOLUTION: Simplify cos(x)cos(y) + sin(x)sin(y)

Question 976584: Simplify cos(x)cos(y) + sin(x)sin(y)
Answer by Edwin McCravy(20056) (Show Source): 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

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