三角函数cos公式有cosA=(b^2+c^2-a^2)/2bc；cosB=(a^2+c^2-b^2)/2ac；cosC=(a^2+b^2-c^2)/2ab等。
三角函数cos数值
cos0°=1、cos15°=（√6+√2）/4、cos30°=√3/2
cos45°=√2/2、cos60°=1/2、cos75°=sin15°、cos90°=0
余弦定理的公式
a b c为三角形3边 A B C为3边所对角
cosA=(b^2+c^2-a^2)/2bc
cosB=(a^2+c^2-b^2)/2ac
cosC=(a^2+b^2-c^2)/2ab
c^2=a^2+b^2-2ab*cosC
cos(a-b)=cosacosb+sinasinb
cos3a
=cos(2a+a)
=cos2acosa-sin2asina
=(2cos^2a-1)cosa-2(1-cos^2a)cosa
=4cos^3a-3cosa
三角函数cos公式
cos(-a) = cos(a)
sin(π/2 - a) = cos(a)
cos(π/2 - a) = sin(a)
sin(π/2 + a) = cos(a)
cos(π/2 + a) = - sin(a)
cos(π - a) = - cos(a)
cos(π + a) = - cos(a)
sin(a + b) = sin(a)cos(b) + cos(α)sin(b)
cos(a + b) = cos(a)cos(b) - sin(a)sin(b)
sin(a - b) = sin(a)cos(b) - cos(a)sin(b)
cos(a - b) = cos(a)cos(b) + sin(a)sin(b)
sin(a) + sin(b) = 2sin[(a + b)/2]cos[(a - b)/2]
sin(a) - sin(b) = 2sin[(a - b)/2]cos[(a + b)/2]
