dy=sintdr+rcostdt
dX=costdr-rsintdt
dr/dt=-asint
dy/dx=[-a(sint)^2+acost+a(cost)^2]/[-asintcost-asintcost-asint]
=-[x^2-y^2+x√（x^2+y^2）]/[2xy+y√（x^2+y^2）]