Prove that the third invariant is equal to the determinant

[blalien]

Homework Statement

This is all in summation notation.
Given a 3x3 matrix A_{ij}, show that det[A]=1/6(A_{ii}A_{jj}A_{kk}+2A_{ij}A_{jk}A_{ki}-3A_{ij}A_{ji}A_{kk})

Homework Equations

I've been told that we're supposed to begin with
det[A]=1/6\epsilon_{ijk}\epsilon_{pqr}A_{ip}A_{jq}A_{kr}

The Attempt at a Solution

I hate to say this, but I have no idea where to start. I would really appreciate if somebody could just give me the first step and push me in the right direction.

 

[Hurkyl]

Definitions are often a good place to start.

 

[blalien]

Never mind, I got it. You set A_{ij} = \delta_{ij} and get the epsilons in terms of the deltas.

 

[Hurkyl]

A_{ij} = \delta_{ij}

Why can you do that?

 

[blalien]

Well, that's not exactly right. But you can work a little index magic and come up with
\epsilon_{ijk}\epsilon_{pqr}=\left| \begin{matrix} \delta_{ip} & \delta_{iq} & \delta_{ir} \\ \delta_{jp} & \delta_{jq} & \delta_{jr} \\ \delta_{kp} & \delta_{kq} & \delta_{kr} \end{matrix} \right|