[mathjiang]

可以了哈：
syms x y21
y21='5*x^2+5';
k2=diff(y21,2);
k1=diff(y21,1);
k1b=k2./(((1+k1.^2)).^(3/2));
k1b=abs(k1b);
x=-11:20;
k1=subs(k1b,'x',-11:20);
k1=double(k1);
nk1=length(k1);
maxk1=max(k1);
[mk1,nk1]=find(k1==maxk1);
plot(x,k1,x(nk1),k1(nk1),'r.')