两式各自平方,得
(sinx)^2+2sinxsiny+(siny)^2=1/4
(cosx)^2+2cosxcosy+(cosy)^2=1/3
cos2a=cosa*cosa-sina*sina=2cosa*cosa-1=1-2sina*sina
两式相加,得 2＋2（sinxsiny+cosxcosy)=7/12
cos(x-y)=sinxsiny+cosxcosy=-17/24＝1－2*sin[(x-y)/2]*sin[(x-y)/2]
得
sin[(x-y)/2]＝正负根号（41/48)
由于cos(x-y)