# Homework Help: Invertible square matrix

1. Feb 28, 2010

### Precursor

1. The problem statement, all variables and given/known data
Show that the square matrix $$2N - I$$ is its own inverse if $$N^{2} = N$$

2. Relevant equations
properties of invertible matrix

3. The attempt at a solution
I really don't know where to start here. I know that $$(2N-I)(2N-I) = I$$, but where do I go on from there?

2. Feb 28, 2010

### VeeEight

You are correct in calculating (2N-I)(2N-I) and using N2=N in your calculations. If (2N-I)(2N-I)=I, then the desired result is achieved, as this implies [(2N-I)]-1=(2N-I).

3. Feb 28, 2010

### Precursor

So the answer is simply $$(2N-I)(2N-I) = I$$? But how did I use $$N^{2} = N$$?

4. Feb 28, 2010

### VeeEight

I just assumed you did :)
The result (2N-I)(2N-I)=I is the desired result. So take (2N-I)(2N-I) and expand it. After you are done that, use N2=N.

5. Feb 28, 2010

### Precursor

ok, so I get:

$$2N^{2} - 2NI - 2NI + I^{2} = I$$

$$2N - 2N - 2N + I = I$$

But this doesn't work out.

6. Feb 28, 2010

### VeeEight

The coefficient on N2 is incorrect. It is not a 2.

7. Feb 28, 2010

### Precursor

Ok I got it. Thanks for the help.

8. Feb 28, 2010

Cheers.