x=ln(1+t²)那么dx/dt=[ln(1+t²)]'=2t/(1+t²)而y=t -arctant所以dy/dt=1 -1/(1+t²)=t²/(1+t²)于是dy/dx=(dy/dt) / (dx/dt)= t/2所以dy/dx(t=1) =1/2
