[SccHarpGirl]

Hey kids,

This is not my first lin alg related post, and probably not my last. This one centers around our assignment which is due on the web. You have as many tries as you like to submit something, but no feedback on how you're doing--you find out if it's right or it's wrong, so as a result if it's incorrect, it could mean that you're way off, or that you made a formatting error but your answer is right.

Anyway, the problems that it's giving me grief about revolve around characteristic polynomials (the polynomial formed by det(A-lambda*I)). I understood the notes, felt it was easy stuff, went to do the problem, and every time I do it I get marked incorrect, but I solve the problem multiple ways and get the same answer.

So I figured I'd run it by the CGR crowd:

A =
-2 -4 0
0 5 -5
3 -4 0

Okay. The formula my prof gave us as an easy way to solve the problem is:
Fa(x) = -x^3 + tracex^2 - (C11+ C22 + C33)x + det(A).

where trace(A) = sum of the diagonal elements
C11....C33 are the cofactors corresponding to those elements
and det(A) = the determinant.

So I get...
trace = 3 (5-2 = 3)
C11 = -20
C22 = 0
C33 = -10
so the sum is -30
and det(A) = 100

so that leaves me with...
-x^3 + 3x^2 + 30x + 100.

I get this answer pretty much no matter what I try. I tried using LaPlace expansion on the 3rd column of [A-Lambda*I] and I obtained the same solution.

But the computer keeps marking me wrong, and I"m not sure what's going on...I tried virtually all sign permutations I could think of, and none of them work either.

Ideas?

[SccHarpGirl]

Nevermind, figured it out (Right answer, wrong formatting).