y-sinx/x-cosx==-cos(x+y/2)/sin(x+y/2)
也看不出如何得到
cosxcos(x+y/2)+sinxsin(x+y/2)=cos(x-y/2)
如何再由此式得到
cos(x+y/2)x+sin(x+y/2)y=cos(x-y/2)
kai 寫到:直線ax+by=c(cosx,sinx) (cosy,siny)在上面故又可寫成y-sinx/x-cosx=siny-sinx/cosy-cosx即cos(x+y/2)x+sin(x+y/2)y=cos(x-y/2)得證