显然
dx/dt=a(1-cost)
dy/dt=a*sint
那么
dy/dx=sint /(1-cost)
继续求二阶导就得到
d(dy/dx)/dt *dt/dx
=[(sint)' *(1-cost) -sint *(1-cost)']/(1-cost)^2 *1/ a(1-cost)
=(cost-1)/(1-cost)^2 *1/ a(1-cost)
= -1/ [a(1-cost)^2]