Given that cos(x)+cos(y)=a\cos(x) + \cos(y) = acos(x)+cos(y)=a and sin(x)+sin(y)=b\sin(x) + \sin(y) = bsin(x)+sin(y)=b, express cos(x−y)\cos(x-y)cos(x−y) in terms of aaa and bbb.
a2+b2−22\frac{a^2 + b^2 - 2}{2}2a2+b2−2
a2+b2−1a^2 + b^2 - 1a2+b2−1
a2+b22\frac{a^2 + b^2}{2}2a2+b2
a2+b2\sqrt{a^2 + b^2}a2+b2