为什么sin2A+sin2B=sin2C得sin(A+B)cos(A-B)=2sinCcosC

问题描述:

为什么sin2A+sin2B=sin2C得sin(A+B)cos(A-B)=2sinCcosC

sin2A+sin2B=sin2C得sin(A+B)cos(A-B)=2sinCcosC 答:因为 sin(A+B)=sinAcosB+sinBcosA cos(A-B)=cosAcosB+sinAsinB sin(A+B)cos(A-B)=(sinAcosB+sinBcosA)(cosAcosB+sinAsinB)整理得 = sin2A+sin2B