Question 1155559: Solving:
5 x-3y = 18
-3 x-3y = -12
Using Substitution Method.
Expressing x as a function of y (from the first equation) x = (18 +3y)/5
Substituting this expression into the second equation:
-3 (18 -3y)/5-3y = -12
Why is this not -3(18+3y)/5-3y = -12 ?
Found 4 solutions by ankor@dixie-net.com, Alan3354, ikleyn, MathTherapy:
Answer by ankor@dixie-net.com(22740)   (Show Source):
You can put this solution on YOUR website! Solving:
5x - 3y = 18
-3x - 3y - -12
:
There is a lot simpler way to do this
-3x -3y = -12
simplify, divide -3
x + y = 4
y = -x+4
Using Substitution Method.
Replace y with (-x+4) in the first equation
5x - 3(-x+4) = 18
distribute the -3
5x + 3x - 12 = 18
8x = 18 + 12
8x = 30
x = 
x = 3.75
find y
y = -x + 4
y = -3.75 + 4
y = .25
:
Check solutions in the 1st equation
5x - 3y = 18
replace x & y
5(3.75) - 3(.25) = 18
18.75 - .75 = 18
Answer by Alan3354(69443)  (Show Source):
You can put this solution on YOUR website! 5 x-3y = 18
-3 x-3y = -12
=================
Substitution is not a good choice here, tho you might be instructed to use it.
====
If you mean:
5x -3y = 18
-3x-3y = -12
---------------------- Subtract
8x = 30
x = 15/4
-----
Sub for x in either equation and find y.
-3 x-3y = -12
x + y = 4
y = 4 - 15/4
y = 1/4
Answer by ikleyn(52783)  (Show Source):
You can put this solution on YOUR website! .
I will do it using substitution method, but I will use a TRICK to avoid using fractions on the way.
Watch my steps carefully.
Express 3y from the first equation 3y = 5x - 18 and then substitute this expression into the second equation, replacing 3y there
-3x - (5x-18) = -12.
Now simplify and find x
- 3x - 5x + 18 = -12
- 8x = - 12 - 18
-8x = - 30
x =  = = 3.75.
Now you substitute this found value of x into either of the two original equations and find y.
--------------
Learn this trick from my post.
There is a lesson in this site, demonstrating this trick in work
- Useful tricks when solving systems of 2 equations in 2 unknowns by the Substitution method
Answer by MathTherapy(10552)  (Show Source):
You can put this solution on YOUR website!
Solving:
5 x-3y = 18
-3 x-3y = -12
Using Substitution Method.
Expressing x as a function of y (from the first equation) x = (18 +3y)/5
Substituting this expression into the second equation:
-3 (18 -3y)/5-3y = -12
Why is this not -3(18+3y)/5-3y = -12?
Looking at the 2 equations, you'll notice that they both have - 3y.
5x - 3y = 18_____- 3y = 18 - 5x ----- Solving for - 3y ----- eq (i)
- 3x - 3y = - 12 ----- eq (ii)
- 3x + 18 - 5x = - 12 ---- Substituting 18 - 5x for - 3y in eq (ii)
- 3x - 5x = - 12 - 18
- 8x = - 30
Now, substitute  for x in eq (i) to find the value of y.
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