[shirleyflh]

谢谢版主,但你的程序我在自己的机器上运行还是有错误,错误是:
?? Error using ==> ./
Matrix dimensions must agree.
On line 5 ==> k1b=k2./(((1+k1.^2)).^(3/2));

同样的还是单步可以得到结果,但运行run就出现上述错误.

原来的程序,我补充一下y
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)
k1=double(k1)
nk1=length(k1)

maxk1=max(k1)
[mk1,nk1]=find(k1==maxk1)

y=[5.*x.^2]
plot(x,y,x(nk1),y(nk1),'r.')