# Homework Help: Adjoints and Determinants Problem

1. Dec 16, 2009

I need help with this problem, i am totally lost. See attachment. Please explain.

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2. Dec 16, 2009

### VeeEight

What have you tried so far? Perhaps use the famous formula that A-1=[1/det(A)]adj(A).
Also note that det(adj(A)) = det(A)n-1 where n is the size of A

3. Dec 16, 2009

### jegues

The adjoint of A is simply the matrix of cofactors.

So if you took matrix A and did an expansion across the 3rd row to compute its determinant what are you really doing?

Well you're taking the entries 3, -1, -1, multiplying each of them by their respected cofactors and summing them to get the determinant.

How can you relate the desired cofactors to the adjoint? (I stated the answer above)