积化和差
sin(a)sin(b) = -1/2*[cos(a b)-cos(a-b)]
cos(a)cos(b) = 1/2*[cos(a b) cos(a-b)]
sin(a)cos(b) = 1/2*[sin(a b) sin(a-b)]
cos(a)sin(b) = 1/2*[sin(a b)-sin(a-b)]
万能公式
sin(a) = [2tan(a/2)] / {1 [tan(a/2)]^2}
cos(a) = {1-[tan(a/2)]^2} / {1 [tan(a/2)]^2}
tan(a) = [2tan(a/2)]/{1-[tan(a/2)]^2}
其它公式
a·sin(a) b·cos(a) = [√(a^2 b^2)]*sin(a c) [其中，tan(c)=b/a]
a·sin(a)-b·cos(a) = [√(a^2 b^2)]*cos(a-c) [其中，tan(c)=a/b]
1 sin(a) = [sin(a/2) cos(a/2)]^2
1-sin(a) = [sin(a/2)-cos(a/2)]^2
其他非重点三角函数
csc(a) = 1/sin(a)
sec(a) = 1/cos(a)
