Question

cos^2a-2cosacosb+cos^2b+sin^2a-2sinasinb+sin^2b

Answer 1

(сosa-cosb)²+(sina-sinb)²=4sin²(a-b)/2*sin²(a+b)/2+4sin²(a-b)/2*cos²(a+b)²=
=4sin²(a-b)/2*[sin²(a+b)/2 + cos²(a+b)/2]=4sin²(a-b)/2