求极值

syms x y;

f=y^2/9+3*x^2*y+9*x^2+y^2+x*y+9;

f1_x=diff(f,x);f2_x=diff(f,x,2);f1_y=diff(f,y);f2_y=diff(f,y,2);f2_xy=diff(f1_x,y);

D=solve(f1_x==0,f1_y==0);

for i=1:length(D.x)

A=subs(f2_x,{x,y},{D.x(i),D.y(i)});

B=subs(f2_xy,{x,y},{D.x(i),D.y(i)});

C=subs(f2_y,{x,y},{D.x(i),D.y(i)});

if B^2-A*C>0

str=sprintf('(%d,%d) 不是极值点',double(D.x(i)),double(D.y(i)));
disp(str);

elseif B^2-A*C<0

if A<0

str=sprintf('(%d,%d) 是极大值,double(D.x(i)),double(D.y(i)))

disp(str);

end

if A>0

str=sprintf('(%d,%d) 是极小值,double(D.x(i)),double(D.y(i)))

disp(str);

end

end

end