1
x^2+y^2-2y*cosθ-sin^2θ=0
==>x^2+y^2-2y*cosθ+cos^2θ-cos^2θ-sin^2θ=0
==>x^2+y^2-2y*cosθ+cos^2θ=cos^2θ+sin^2θ=1
==>x^2+(y-cosθ)^2=1
即圆心为（0,cosθ）,半径为1