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# inverse watch

1. hi, apparently the inverse for P is as shown,

but i get

[10, -5, -1][-13, 5, 1][-6,3,0]

(with each square bracket being a row).

can anyone tell me where i'm going wrong? thanks
2. Firstly, when you multiply P.P^-1 as given, do you get I?

If so, then P^-1 must be right.

If your inversion went wrong, then it's hard to see why without your working. I hate to invert matrices in public, because I make mistakes.
3. (Original post by ian.slater)
Firstly, when you multiply P.P^-1 as given, do you get I?

If so, then P^-1 must be right.

If your inversion went wrong, then it's hard to see why without your working. I hate to invert matrices in public, because I make mistakes.
i used this

http://www.wikihow.com/Inverse-a-3X3-Matrix

so M11 = (2x5)-(0x-1) = 10

M12 = (1x5)-(0x0) = 5

M13 = (1x-1)-(2x0) = -1

until i got all three rows, then did the whole

+ - +
- + -
+ - +

thing, and multiplied it with the

but none of my answers seem right!

also i don't know what I should be, i use , if this has any connection?
4. At some stage you have to transpose the matrix of minors/cofactors. Your rows should be columns, so it looks like you haven't transposed.

Does that sort it?
5. Also, when you say p^-1.A.P = D I think of diagonalising a matrix; eigenvectors and so on .. is that part of your question?

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