X=arctantdx/dt=1/(1+t^2)y=ln(1+t2)dy/dt=2t/(1+t^2)dy/dx=(dy/dt)/(dx/dt)=2td2y/dx2=d(dy/dx)/dx=2dt/dx=2/(dx/dt)=2(1+t^2)而:X=arctant,t=tanx所以:d2y/dx2=2(1+t^2)=2+2(tanx)^2