x=t+t^2,y=cost
所以
dx/dt=1+2t,dy/dt= -sint
于是
dy/dx=(dy/dt) / (dx/dt)= -sint/(1+2t)
而
d^2y/dx^2
=(dy/dx)/dt *dt/dx
=[-sint/(1+2t)]' /(1+2t)
=[-cost*(1+2t)+2sint]/(1+2t)^3