Problema Solution
I want to know how you simplify this expression: cos(4x + 5y) + cos(4x - 5y)
Answer provided by our tutors
cos (a+b)= cos a cos b - sin a sin b
cos (a-b)= cos a cos b + sin a sin b
we use these formulas:
cos(4x + 5y) + cos(4x - 5y)=
(cos 4x* cos 5y- sin 4x * sin 5y)+ (cos 4x * cos 5y + sin 4x *sin 5y)= 2 (cos 4x* cos 5y)