# How do I find the eigenvalue given unknown rows & eigen vect

Tags:
1. Oct 21, 2016

### Razberryz

1. The problem statement, all variables and given/known data
Consider the following matrix A (whose 2nd and 3rd rows are not given), and vector x.

A =
4 4 2
* * *
* * *
x =
2
-1
10

Given that x is an eigenvector of the matrix A, what is the corresponding eigenvalue?

2. Relevant equations

3. The attempt at a solution

4−λ 4 2
a b−λ c
d e f−λ=

det (λI3-3) =

−λ3+b+f+4×λ2+4×a−4×b+2×d+c×e−b×f−4×f×λ+−2×b×d+4×c×d+2×a×e−4×c×e−4×a×f+4×b×f

Don't know if I'm on the right track. Trying to find the roots of this characteristic equation, but there's so many unknowns.

2. Oct 21, 2016

### Staff: Mentor

What is a eigenvector?

3. Oct 21, 2016

### Razberryz

An eigenvector of A is a non-zero vector x such that Ax = λx

4. Oct 21, 2016

### Staff: Mentor

So. Have you already computed Ax?

5. Oct 21, 2016

### Razberryz

Yes

24 a2-b+c10 d2-e+f10

6. Oct 21, 2016

### Razberryz

I have 5 minutes left to answer the question. Would you mind helping me with the answer, and then you can explain it to me?

7. Oct 21, 2016

### Staff: Mentor

I assume, this should be a column vector. Now what says the equation $Ax = \lambda x$ in the first coordinate?

8. Oct 21, 2016

### Razberryz

Sorry? You mean 24?

9. Oct 21, 2016

### Razberryz

I got it! Thanks!

10. Oct 21, 2016

### Staff: Mentor

I mean the first coordinate is 24 on the left and $\lambda x_1$ on the right, yes.

11. Oct 21, 2016

### Razberryz

I figured it out, thanks a bunch!