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# FP4 Determinants HELP! watch

1. I know I've been completely stupid, left almost all my revision until the night before.

I do NOT understand determinants, and the textbook is awful.

Please can someone give me a quick explanation?
2. FP4??

And determinants of a 2 by 2 matrix??

If so, for a matrix the determinant is

If 3 by 3,

then for a

we do
3. if you have the following matrix:

[ A B C ]
[ D E F ]
[ G H I ]

the determinant is A[EI - FH] - B[EI - FG] + C[DH - EG].

if you're running out of time, just remember that algorithm (or write i down somewhere ).
4. I think for the sake of A-level maths there isn't much to explain. The only thing I can think of is knowing that a matrix is singular (i.e. Has no inverse) if and only if it has determinant zero. Do you learn the minors/cofactors method at A-level to work out an inverse of a 3x3 matrix? If so that's another reason to know about determinants.

Just learn the formulae. The 2x2 case is just ad-bc; the 3x3 case just needs you to remember the method, which in all fairness is hardly difficult.

I seem to remember quite a few questions coming up where you'd be given a matrix, say

[ a 1 7 ]
[ 3 2a 9]
[16 6a 2]

and having to work out the possible values of a given that the determinant is 42.

Why not just go through past papers and pick out all the determinant questions? Papers should be readily available n the exam board website, especially if you're on AQA.
5. I was sure it only went up to FP3
6. AQA I think goes to FP4
7. (Original post by J-E-N-O-V-A)
I was sure it only went up to FP3
AQA has an FP4.
8. Several methods... The best for a 3x3 is expansion by first row or the diagnol lines thing (can't recall the name). Anything bigger than 3x3 and you may want to use dodgsons condensation method.
9. My method for 3x3 matrices works a treat. Say you have the matrix , the determinant can be found by first summing the products of the diagonals from top-left to bottom-right; then subtracting the products of the diagonals from top-right to bottom-left. That is, . It's great fun... it's really just an expansion of
10. Thank-you all, I think I just about understand it now. Wish me luck for tomorrow!

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Updated: January 26, 2009
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