和差化積公式記憶口訣

和差化積需同名，變量置換要記清；假若函數(shù)不同名，互余角度換名稱。簡記為：S+S=2S·C，S-S=2C·S，C+C=2C·C，C-C=-2S·S。

積化和差

sina*cosb=(sin(a+b)+sin(a-b))/2

cosa*sinb=(sin(a+b)-sin(a-b))/2

cosa*cosb=(cos(a+b)+cos(a-b))/2

sina*sinb=-(cos(a+b)-cos(a-b))/2

和差化積

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)