sin(A+B)=sinAcosB+cosAsinB
sin(A-B)=sinAcosB-sinBcosA
cos(A+B)=cosAcosB-sinAsinB
cos(A-B)=cosAcosB+sinAsinB
tan(A+B)=(tanA+tanB)/(1-tanAtanB)
tan(A-B)=(tanA-tanB)/(1+tanAtanB)
cot(A+B)=(cotAcotB-1)/(cotB+cotA)
cot(A-B)=(cotAcotB+1)/(cotB-cotA)
cos2a=(cosa)^2-(sina)^2=2(cosa)^2 -1=1-2(sina)^2 sin2A=2sinA*cosA
tan3a=tana*tan(π/3+a)*tan(π/3-a)
tan(A/2)=√((1-cosA)/((1+cosA))
tan(A/2)=-√((1-cosA)/((1+cosA))
cot(A/2)=√((1+cosA)/((1-cosA))
cot(A/2)=-√((1+cosA)/((1-cosA))
tan(A/2)=(1-cosA)/sinA=sinA/(1+cosA)
在二角和的公式中令两个角相等(B=A),就得到二倍角公式.
sin(A+B)=sinAcosB+cosAsinB
cos(A+B)=cosAcosB-sinAsinB
--->cos2A=(cosA)^2-(sinA)^2=(1-(sinA)^2-(sinA)^2=1-2(sinA)^2=2(cosA)^2-1.
tan(A+B)=(tanA+tanB)/(1-tanAtanB)
--->tan2A=2tanA/[1-(tanA)^2]
--->sin(x/2)=+'-√[(1-cosx)/2] 符号由(x/2)的象限决定,下同.
--->cos(x/2)=+'-√[1+cosx)/2]
tan(x/2)=+'-√[(1-cosx)/(1+cosx)].
又tan(x/2)=sin(x/2)/cos(x/2)
=2[sin(x/2)]^2/[2sin(x/2)cos(x/2)]