Error using solve function

solve function for code

Gives the error: unable to find explicit solution.

        x1 = input('Input volume of tank in gallons: ');
        x2 = x1/7.48;
        syms int_l;
        
cost = 812.745*(2*int_l.^2+4.*(int_l+0.5)*(y./(int_l.^2)+1));
diff_cost = diff(cost);

answ = solve(diff_cost == 0);
int_l = answ(isAlways(answ>0));
int_l = double(int_l);
int_h = x2/(int_l.^2);
ext_l = int_l + 1;
ext_h = x2./(int_l.^2)+1;
        
disp('----------------------------------------------------------------')
disp(['Length of the internal height in feet will be: ' num2str(int_l)])
        
        
        

Answer 1

[Steven Lord]

You have two variables in your cost function, y and int_l. Is y symbolic or a fixed value? If symbolic, with respect to which variable do you want to differentiate the cost?

syms int_l y;
        
cost = 812.745*(2*int_l.^2+4.*(int_l+0.5)*(y./(int_l.^2)+1));
diff_cost = diff(cost)

diff_cost = 

diff_cost_y = diff(cost, y)

diff_cost_y = 

diff_cost_int_l = diff(cost, int_l)

diff_cost_int_l = 

The expressions diff_cost and diff_cost_y depend only on int_l but diff_cost_int_l depends on both int_l and y. If you're trying to solve diff_cost_int_l == 0, with respect to which variable are you trying to solve?

solve(diff_cost_int_l, int_l)

ans = 

solve(diff_cost_int_l, y)

ans = 

[Muhammad Rohaan]

it seems that i was taking a wrong variable. Thanks a lot.