# Help with this math prob please, test tomorrow!!

Discussion in 'Off Topic [BG]' started by bassist 4 life, Dec 13, 2006.

1. ### bassist 4 life

Aug 1, 2004
Bowling Green, Ohio
Evaluate the determinant of :

| -1 3 4 |
| 0 5 1 |
| 6 -2 3|

The answer is -119. I just don't know how you get -119

you can use either expression by minors or diagonals to solve it

2. ### karrot-xBanned

Feb 21, 2004
Omicron Persei 8

Mar 8, 2002
YYC
First row reduce it to get a column of | "number" 0 0 |, it'll make your life easier.

| -1 3 4 |
| 0 5 1 |
| 0 16 27|

You can find the determinant by cofactor, so you can use any row or any column to do this. Pick the first row since it has the most number of 0. Since -1 is the only number, the rest will be zeros (any number multipled by 0 is 0). Dont forgot the negatives, since -1 is at the (1,1) position on the grid we have to multiply the determinant by (-1)^(1+1) to make sure the sign it correct. Since (-1)^2 is 1 we dont have to worry about it.

so make a new matrix..

(-1) | 5 1 |
| 16 27|

(-1)[(5)(27)-(16)(1)] = -119

Does it help? Or have I just confused you? I just had my Linear algebra final yesterday. Good luck with yours!

4. ### MCBTunes

Dec 17, 2004
he's 14 and I cant help him with his math? Now I feel low... but after reading Jade's post I think I'm remembering a little... but still...

5. ### 98dvl

Jan 31, 2002
USA
Don't worry, you'll never use that stuff in real life anyway.

6. ### karrot-xBanned

Feb 21, 2004
Omicron Persei 8
Unless your job depends on it.

Mar 8, 2002
YYC
Dorry worry, they have computers to do this stuff for us.

8. ### karrot-xBanned

Feb 21, 2004
Omicron Persei 8
Unless the computer breaks.

9. ### fishstix

Feb 28, 2006
Wisconsin
Recopy the first two columns to the right side of the determinant. Then add the products of the diagonals from top left to bottom right. From that sum take away the sum of the products of the diagonals from bottom left to top right.

EX:
| -1 3 4 |
| 0 5 1 |
| 0 5 1 |

becomes

| -1 3 4 | -1 3
| 0 5 1 | 0 5
| 6 -2 3| 6 -2

then...

[(-1x5x3)+(3x1x6)+(4x0x-2)]-[(6x5x4)+(-2x1x-1)+(3x0x3)]

If you are only having to work up to a 3 by 3 det., you'll be fine with this method. My H.S. students in Alg. 2 only have to work through 3 by 3's and thus this explination is as far as we have to go. There are more powerful generalized methods out there but save those for Pre-calc and beyond.
Good luck,
Fishstix

10. ### bassist 4 life

Aug 1, 2004
Bowling Green, Ohio
Thanks for the help guys,

btw I just learnt how to solve matrices in my calculator , this iwll make things a lot easier.

11. ### Eric Perry

Apr 13, 2001
berkeley, ca
i had to solve for the determinant of a 3x3 matrix for my quantum mechanics final.

13. ### Minger

Mar 15, 2004
Rochester, NY
I'm currently in AP Statistics na dAP Calculus...and have never seen this before in my life

14. ### Neb MaroI don't think, but I still am.

Oct 20, 2006
So. Cali
I despise higher math. To think I have Algebra 2, Geometry, Calculus, and Physics to look forward to.

15. ### Jeff MooteSupporting Member

Oct 11, 2001
Well, then it's something to look forward to!

16. ### 98dvl

Jan 31, 2002
USA
Like I said... Real life != school

17. ### karrot-xBanned

Feb 21, 2004
Omicron Persei 8
Unless ....

18. ### Anesthesia

Jun 4, 2005
Perth Australia
just write -119 and pretend you did it.

19. ### Poop-LoopsBanned

Mar 3, 2006
Auburn, Washington
I didn't do 3x3 determinants till last spring. You must be really ahead of the curve... or whereever you live is really ahead of the curve.

Or I don't know, but it's cool that you're learning that now.