18°=π/1063°=7π/20
cos π/10 * cos 7π/20 + sin π/10 * sin 7π/20 = cos (π/10 - 7π/20)по формуле cos a*cos b+sin a*sin b=cos(a-b)cos (π/10 - 7π/20) = cos (2π/20-7π/20)=cos(-5π/20)=cos(-π/4)=cos(7π/4)=√2/2
18°=π/10
63°=7π/20
cos π/10 * cos 7π/20 + sin π/10 * sin 7π/20 = cos (π/10 - 7π/20)
по формуле cos a*cos b+sin a*sin b=cos(a-b)
cos (π/10 - 7π/20) = cos (2π/20-7π/20)=cos(-5π/20)=cos(-π/4)=cos(7π/4)=√2/2