Question

Cos24°×cos31°-Sin24°×sin31°-cos55°

Answer 1

cos24°cos31°-sin24°sin31°-cos55°=cos(24°+31°)-cos55°=cos55°-cos55°=0

Answer 2

Почему?

Answer 3

Формула cos(t)*cos(s)-sin(t)*sin(s)=cos(t+s)