Question

Use Cramer's Rule to solve: x+2y+5z=10 , 3x+4y-z=10 , x-y-z=-2 HELP!!!!

Answer 1

cramers rule uses determinants, do you remember these?

like this:
1 2 5 10
3 4 -1 10
1 -1 -1 -2

Answer 2

Is there a way you could do them on a calculator? I don't remember this!

Answer 3

There is if you have like an $120 dollar calculator..

Answer 4

check this page, it will help i think

http://www.purplemath.com/modules/cramers.htm

Answer 5

I have a TI-84 plus. It keeps saying invalid dimensions.

Answer 6

can 84's do matrixes? I have an 89... so im not sure about 84s

Answer 7

Yeah, they can. You press 2nd then x^-1 go to MATH it has det so I put det([A]) and it just keeps saying invalid dimensions. I guess I should learn this by hand instead.

Answer 8

ohhh, i get what you are saying.
when you enter the matrix it needs to have the same number of rows as there are columns.

Answer 9

so first enter a 3x3 matrix with just the numbers from the left hand side of the equations. and get that value
so enter this:
1 2 5
3 4 -1
1 -1 -1

Answer 10

this we will call "D" when you get the value

Answer 11

then do this one:
10 2 5
10 4 -1
-2 -1 -1
and we will call it "Dx" we call it Dx because I replace the 'x' column with the 'answer' column

Answer 12

Okay! I got -36

Answer 13

for D

Answer 14

so D=-36? ya? ok!

Answer 15

great, now do the Dx like I said

Answer 16

I got Dx=-124

Answer 17

excellent, idk if those are correct but we will assume they are.

now the next step is to do this:

Dx/D

Answer 18

the value you get for this will be what 'x' is equal to

Answer 19

so -36/-124 = 0.29....

Answer 20

or 9/31

ya?

Answer 21

I was reading that I should get -36 for Dx also. My calculator says -124...?

Answer 22

I thought we got -36 as D?

Answer 23

Yeah, x was supposed to equal 1. I just don't understand how you get -36 for Dx too?

Answer 24

so if x=1, then it would have to be..
Dx=-36
D=36?

Answer 25

Right. They just didn't say it was equal to 1. I had to figure that out. I'm looking at a similar problem. How would I find Dx so I can know if it's -36??

Answer 26

x=1 is correct, I just looked it up

Answer 27

try doing Dx again in your calc, it should be like this:

10 2 5
10 4 -1
-2 -1 -1

Answer 28

I gotta take off, but here are some helps
Dy = -96
Dz = -36

good luck!

Answer 29

Okay! I got it. Thanks!! :)