Cos24°×cos31°-Sin24°×sin31°-cos55°

Question

Cos24°×cos31°-Sin24°×sin31°-cos55°

Answer 1

Cos24°cos31°-sin24°sin31°-cos55°=cos(24°+31°)-cos55°=cos55°-cos55°=0

Comments

Почему?

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