To Mohsen
Actually, I don't know what exactly you want to do. But There are some problems in your code such as variable dimension(size), matrix(array) calculation...
check this out. hope this will be helpful for you.
F = [-40:20:40];
x = linspace(0.1,0.8,length(F));
y = linspace(0.1,0.4,length(F));
sigma_p = @(x,y) F./(.8.*x.*y);
sigma_mx = @(x,y) (F.*y)./(1./12.*(x).*(y).^3);
sigma_my = @(x,y) (F.*x)./(1./12.*(y).*(x).^3);
sigma_net = @(x,y) sigma_p(x,y) + sigma_mx(x,y) + sigma_my(x,y);
figure
plot3(x, y, sigma_net(x,y))
