餘弦函式的相乘公式主要包括以下幾種:
對於任意角度A,B,C,有:
cosA = (b^2 + c^2 - a^2) / 2bc
cosB = (a^2 + c^2 - b^2) / 2ac
cosC = (a^2 + b^2 - c^2) / 2ab
對於特殊角度,有:
cos30° = √3 / 2
cos60° = 1 / 2
cos90° = 0
cos0° = 1
cos(180° - x) = -cosx
cos(90° - x) = sinx
對於角度的兩倍,有:
cos2x = (cosx)^2 - (sinx)^2
cos2x = 2(cosx)^2 - 1
cos2x = 1 - 2(sinx)^2