求参数方程dy/dx的二阶导数,x=acost,y=bsint

dy/dt=bcost
dx/dt=-asint
dy/dx=-(b/a) *cott
d^2y/dx^2=d(dy/dx)/dx={d(dy/dx)/dt}/(dx/dt)=(b/a) *csc^2 t/-asint=-(b/a^2) *csc^3 t