Expert: Sherman D. - 10/15/2008

Question
QUESTION: -5x + 3y + Z = -15
10x + 2y + 8z =18
15x + 5y + 72 =9
Please help... My daughter is having trouble sloving this problem.  Thank you

ANSWER: -5x + 3y + Z = -15
10x + 2y + 8z = 18
15x + 5y + 72 = 9

just to check, did you mean 72, 72z, or 7z. Did you write all of this down correctly.

---------- FOLLOW-UP ----------

QUESTION: It is 7z... Sorry

Answer
-5x + 3y + z = -15
10x + 2y + 8z = 18
15x + 5y + 7z = 9


-5x + 3y + z = -15
z = 5x - 3y - 15

10x + 2y + 8z = 18
10x + 2y + 8(5x - 3y - 15) = 18
10x + 2y + 40x - 24y - 120 = 18
50x - 22y = 138
25x - 11y = 69

15x + 5y + 7z = 9
15x + 5y + 7(5x - 3y - 15) = 9
15x + 5y + 35x - 21y - 105 = 9
50x - 16y = 114
25x - 8y = 57

25x - 11y = 69
25x - 8y = 57

multiply top by -1

-25x + 11y = -69
25x - 8y = 57

3y = -12
y = -4

25x - 11(-4) + 69
25x + 44 = 69
25x = 25
x = 1

-5(1) + 3(-4) + z = -15
-5 - 12 + z = -15
-17 + z = -15
z = 2

x = 1
y = -4
z = 2

you can also use www.quickmath.com, click on Solve under Equations, then click Advanced, type it in like i have, then make sure you have x, y, and z for the variables on the right, then click solve.

by the way, sometimes you have to work it other ways until the answers turn out right.