dx/dt=[(1+t^2)3a-3at*2t]/(1+t^2)^2=3a(1-t^2)/(1+t^2)^2
dy/dt=[(1+t^2)6at-3at^2*2t]/(1+t^2)^2=6at/(1+t^2)^2
所以dy/dx=(dy/dt)/(dx/dt)=2t/(1-t^2)
dx/dt=[(1+t^2)3a-3at*2t]/(1+t^2)^2=3a(1-t^2)/(1+t^2)^2
dy/dt=[(1+t^2)6at-3at^2*2t]/(1+t^2)^2=6at/(1+t^2)^2
所以dy/dx=(dy/dt)/(dx/dt)=2t/(1-t^2)