Originally Posted by

**B-Ren** I have a test tomorrow, and I can do all the problems on the review sheet except these two:

$\displaystyle x+y-3z=9$

$\displaystyle y+4z=6$

$\displaystyle y-4z= 12$

add the last two equations ...

2y = 18 ... you should be able to get x and z from here

And:

$\displaystyle 2/x-5/y=4$

$\displaystyle 3/x+4/y= 10$

I know for this one I do this:

$\displaystyle A=1/x$

$\displaystyle B=1/y$

$\displaystyle 2A-5B=4$

$\displaystyle 3A+4B=10$

multiply the first equation by -3, the second by 2 , then sum ...

-6A + 15B = -12

6A + 8B = 20

---------------

23B = 8

B = 1/y = 8/23 ... y = 23/8

2A - 5(8/23) = 4

46A - 40 = 92

46A = 132

A = 1/x = 132/46 = 66/23 ... x = 23/66