令t=sinxcosx
x+y=1+2t,t=(x+y-1)/2,
xy=t(1+2t)=(x+y-1)/2*(x+y)=(x^2+y^2+2xy-x-y)/2=(x^2+y^2-x-y)/2+xy
∴x^2+y^2-x-y=0
即(x-1/2)^2+(y-1/2)^2=1/2,为一圆