半角公式sin2X=2sinXcosX；cos2X=(cosX)方-(sinX)方；tan2X=2tanX/(1-(tanX)方)；cos2X=1-2sinX方=2(cosX)方-1

　　和差化积sinX+sinY=2sin[(X+Y)/2]cos[(X-Y)/2]；

　　sinX-sinY=2cos[(X+Y)/2]sin[(X-Y)/2]

　　cosX+cosY=2cos[(X+Y)/2]cos[(X-Y)/2]

　　cosX-cosY=-2sin[X+Y)/2]sin[(X-Y)/2]

　　积化和差sinXcosY=1/2[sin(X+Y)+sin(X-Y)]

　　cosXsinY=1/2[sin(X+Y)-sin(X-Y)]

　　万能公式sin2X=2tanX/(1+(tanX)方)

　　cos2X=(1-(tanX)方)/(1+(tanX)方)