cos(x+y)-cosx如何等于-2sin[(2x+y)/2]sin(y/2),_数学解答

cos(x+y)-cosx如何等于-2sin[(2x+y)/2]sin(y/2),

人气：368 ℃　时间：2020-05-20 13:33:40

解答

x+y=(2x+y)/2+(y/2) x=(2x+y)/2-(y/2)cos(x+y)-cosx=cos[(2x+y)/2+(y/2)]-cos[(2x+y)/2-(y/2)]=cos[(2x+y)/2]*cos(y/2)-sin[(2x+y)/2]*sin(y/2)-cos[(2x+y)/2]*cos(y/2)-sin[(2x+y)/2]*sin(y/2)=-2sin[(2x+y)/2]sin(...