вычислить cos18 градусов cos63 градуса + sin18 градусов sin63 градуса

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

=cos (18-63) = cos (-45) = cos315=7 п/4