∵sinx+siny=1/2∴(sinx+siny)²=1/4即:sin²x+2sinxsiny+sin²y=1/4.(1)令cosx+cosy=t则t²=cos²x+2cosxcosy+cos²y.(2)(1)+(2):t²+1/4=2+2(sinxsiny+cosxcosy)即:t²=2cos(x-y)...