sinx+cosy=t.1
cosx+siny=√2/2.2
1式平方+2式平方得
2+2sinxcosy+2cosxsiny=t^2+1/2
2sin(x+y)=t^2-3/2
sin(x+y)=(2t^2-3)/4
所以-1≤(2t^2-3)/4≤1
-1/2