'subs' not working as i want

Hello guys,

I have this equation -x^2/2 + 2*x - (5*I)/2 and I'm trying to replace the letter x with a the matrix A and the letter I with the matrix B.

A = 0 1 0

0 0 1

2 -5 4

B = eye(3)

The problem is that when I write subs(eq, x, A) and subs(eq, I, eye(3)) I get a very big matrix, and this is not the answer.

[YT]

It makes sense that your result will be "a very big matrix". subs(eq, x, A) will give you a 3x3 matrix because you substitute x for a 3x3 matrix. With subs(eq, I, eye(3)) you are trying to substiute an identity matrix in your previous subsituted matrix. So what do you expect the result to be then?

[Fernando Pérez Lara]

Yes, you are right, but I didn't know how to use the subs. One guy gave me the correct answer. Thank you for your contribution

Answer 1

[Walter Roberson]

Do both subs at the same time

subs(eq, {x, I}, {A, eye(3)})

[Fernando Pérez Lara]

It works! Thank you very much.