## 'subs' not working as i want

### Fernando Pérez Lara (view profile)

on 8 Dec 2018
Latest activity Commented on by Fernando Pérez Lara

on 9 Dec 2018

### Walter Roberson (view profile)

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

### YT (view profile)

on 8 Dec 2018
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

### Fernando Pérez Lara (view profile)

on 9 Dec 2018
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

### Walter Roberson (view profile)

on 8 Dec 2018

Do both subs at the same time
subs(eq, {x, I}, {A, eye(3)})

#### 1 Comment

Fernando Pérez Lara

### Fernando Pérez Lara (view profile)

on 9 Dec 2018
It works! Thank you very much.