式子P=cosθ-sinθ两边同乘P,
P^2=Pcosθ-Psinθ
因为P^2=X^2+Y^2
X=Pcosθ
Y=Psinθ
则
X^2+Y^2=X-Y
即
(X-1/2)^2+(Y+1/2)^2=1/2
则圆心为(1/2,-1/2),半径为1/根2