[NEED HELP!] Cần giúp về lập trình matlab!

function n1b3

clc

syms x y real

f=input('nhap ham f(x,y)= ');

[a b]=solve([char(diff(f,'x')) '=0'],[char(diff(f,'y')) '=0']); % giai dao ham cap 1

a=double(a);

b=double(b);

% tinh dao ham cap 2

A=diff(f,2,x);

B=diff(f,x);B=diff(B,y);

C=diff(f,2,y);


cd=zeros(0); ct=zeros(0);

zcd=zeros(0); zct=zeros(0);

n=size(a,1);i=1;

while i<=n;

    x=a(i);y=b(i);

    sA=eval(A);sB=eval(B);sC=eval(C); %tim A,B,C

    delta=(sA*sC-sB^2); %tinh delta

    delta=double(delta);

    if delta > 0

	   if sA > 0 % A > 0 la cuc tieu

		  ct=[ct;a(i) b(i)]; zct=[zct;eval(f)];

		  i=i+1;

	   elseif sA < 0 % A > 0 la cuc dai

		  cd=[cd;a(i) b(i)]; zcd=[zcd;eval(f)];

		  i=1+i;

	   else

		  a(i)=[];b(i)=[];

		  n=n-1;

	   end

    else

	   a(i)=[];b(i)=[];

	   n=n-1;

    end

end


if size([zcd;zct],1)>= 2 % ve hinh voi 2 cuc tri tro len

    [x,y]=meshgrid(min(a)-abs(max( a )-min(a))/5:.1:max(a)+abs(max(a)-min(a))/5,min(b)-abs(max(b)-min(b))/5:.1:max(b)+abs(max(b)-min(b))/5);

    f=char(f);f=strrep(f,'^','.^');f=strrep(f,'*','.*'  );f=eval(f);

    [x y f]=khu(x,y,f);

    set(surf(x,y,f),'facecolor','b','edgecolor','non',  'facealpha',.3)

    hold on

    ctri(cd,ct,zcd,zct)

elseif size([zcd;zct],1)== 1 % ve hinh voi 1 cuc tri

    [x,y]=meshgrid(a-2:.1:a+2,b-2:.1:b+2);

    f=char(f);f=strrep(f,'^','.^');f=strrep(f,'*','.*'  );f=eval(f);

    [x y f]=khu(x,y,f);

    set(surf(x,y,f),'facecolor','b','edgecolor','non',  'facealpha',.3)

    hold on

    ctri(cd,ct,zcd,zct)

else % khong co cuc tri

    disp('f khong co cuc tri chat, con co cuc tri không chat hay khong thi chiu ' )

    [x,y]=meshgrid(-2:.1:2);

    f=char(f);f=strrep(f,'^','.^');f=strrep(f,'*','.*'  );f=eval(f);

    [x y f]=khu(x,y,f);

    set(surf(x,y,f),'facecolor','b','edgecolor','non',  'facealpha',.3)

end

rotate3d on

hold off

xlabel('truc x')

ylabel('truc y')

zlabel('truc z')

end


function ctri(cd,ct,zcd,zct) % chuong trinh ve cuc tri

cd=double(cd);zcd=double(zcd);

for i=1:size(zcd,1)

    disp([' f co cuc dai chat: ' '(' num2str(cd(i,1)) ',' num2str(cd(i,2)) ',' num2str(zcd(i)) ')'])

    [x,y]=meshgrid(cd(i,1)-0.2:.05:cd(i,1)+0.2,cd(i,2)-0.2:.05:cd(i,2)+0.2);

    z=zcd(i)+x.*0+y.*0;

    set(surf(x,y,z),'facecolor','r','edgecolor','non')

    text(cd(i,1),cd(i,2),zcd(i)+.1,['cuc dai (' num2str(cd(i,1)) ',' num2str(cd(i,2)) ',' num2str(zcd(i)) ')'])

end

ct=double(ct);zct=double(zct);

for i=1:size(zct,1)

    disp([' f co cuc tieu chat: ' '(' num2str(ct(i,1)) ',' num2str(ct(i,2)) ',' num2str(zct(i)) ')'])

    [x,y]=meshgrid(ct(i,1)-0.2:.05:ct(i,1)+0.2,ct(i,2)-0.2:.05:ct(i,2)+0.2);

    z=zct(i)+x.*0+y.*0;

    set(surf(x,y,z),'facecolor','r','edgecolor','non')

    text(ct(i,1),ct(i,2),zct(i)-.1,['cuc tieu (' num2str(ct(i,1)) ',' num2str(ct(i,2)) ',' num2str(zct(i)) ')'])

end

end

function [x y f]=khu(x,y,f) % chuong trinh loai bo cac diem khong ton tai cua ham f

f=double(f);

for i=1:length(x)

    for j=1:length(y)

	   if ~isreal(f(i,j))

		  f(i,j)=NaN;x(i,j)=NaN;y(i,j)=NaN;

	   end

    end

end

end