syms t
x=1-t^2;y=t-t^3;
dy=diff(y)/diff(x);
dyy=diff(dy)/diff(x);
dyy=simplify(dyy) % d^2y/dx^2
ezplot(x,y,[-1.5 1.5])
subs(dyy,sym(sqrt(2)/2)) %t=√2 /2处,二阶导数值