>>309
dx/dt=-cost , dy/dt=1+sint , dy/dx=(dy/dt)/(dx/dt)=-(1+sint)/cost

d^2y/dx^2 = (d/dx)(dy/dx) = (dt/dx)(d/dt)(dy/dx)
= {1/(dx/dt)}*(d/dt){-(1+sint)/cost}
=(-1/cost)*{-(cost)^2-(1+sint)*sint}/(cost)^2
=(1+sint)/(cost)^3
=(1+sint)/{(1+sint)(1-sint)cost}
=1/{(1-sint)cost}