Posted by **ann** on Thursday, July 26, 2007 at 10:25pm.

solve the equation: First: x + 3y = 5 Second: 3x - y = 5

should i use substitution or add them?

"First" + 3 times "Second" eliminates y:

x + 3y + 3*(3x - y) = 5 + 3*5 --->

10 x = 20 ---->

x = 2

Inserting in "First" gives:

2 + 3 y = 5 --->

3 y = 3 ---->

y = 1

This problem is a bit too easy to practice solving equations. You should try to practice yourself by giving three variables x1, x2, x3 arbitrary values, say

x1 = 4, x2= 13, x3 = 3

and then evaluate three different linear functions of the x1, x2 and x3, e.g.

x1 + 2 x2 - 7 x3 = 9

2 x1 - x2 + x3 = 6

-x1 + 5 x2 - 3 x3 = 52

Then try to solve these three equations. If you do it correctly you should obtain the values you used for x1 x2 and x3 for the solution. It can happen, though, that you find more solutions than the values you started out with.

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