Why is matlab not evaluating equation correctly

    x1=sym('x1');
    x2=sym('x2');
    a=sym('a');

    fx0(x1,x2) = 10*((x1)+3)^2+(x2)^2;

    sp=[-1 , 1];

    fx0d(x1,x2) = gradient(fx0);

    dsd = -1*fx0d(sp(1,1),sp(1,2));

    dsd1 = fx0(sp(1,1)+a*dsd(1,1),sp(1,2)+a*dsd(2,1))

When I run this code the variable dsd1 is not correct. The correct answer is 10(-40a+2)^2+(1-2a)^2 but matlab returns (2*a - 1)^2 + 10*(40*a - 2)^2 and I have no idea why.

Answer 1

[Star Strider]

When I use your code, and then also add at the end of that code:

Q = 10*(-40*a+2)^2+(1-2*a)^2;
Q = simplify(Q, 'steps', 10)

and run it, I get as the result:

dsd1 =

(2*a - 1)^2 + 10*(40*a - 2)^2

Q =

(2*a - 1)^2 + 10*(40*a - 2)^2

Unless I’m missing something, those look a lot alike!

[Josh McCaffrey]

Thanks! I think I need to take a break.. lol

[Star Strider]

My pleasure!

I know the feeling.