x = r*cosθ, y = r*sinθ、dx=-r*sin(θ)dθ=-ydθ、dy=r*cos(θ)dθ=xdθ、
xdy-ydx=x^2dθ+y^2dθ=dθ(x^2+y^2)=r^2dθ、xdx+ydy=-xydθ+xydθ=0