Помогите решить V18cos^2 7п/8-v18sin^2 7п/8

Question

Помогите решить V18cos^2 7п/8-v18sin^2 7п/8

Answer 1

√18(cos²7π/8-sin²7π/8)=√18*cos7π/4=√18*cos(2π-π/4)=√18*cosπ/4=
=√18*√2/2=√36/2=6/2=3