Substituting (URGENT!!)

PreAlgebraDummy15

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Nov 6, 2005
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38
ok now I'm doing substituting:

x+y=5
5x-3y=17
________
-4x-2y=-12



thats what I got so far (the bottom equation) if anyone can tell me if it's right and maybe help me with the next step that would be great! Thanks
 

Gene

Senior Member
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Oct 8, 2003
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Well firstly the bottom equation is wrong. When you subtract -3y from y you get +4y.
1) x+y=5
2) 5x-3y=17
________
3) -4x+4y=-12
4) -x+y=-3

If you then add 1) & 4) you get
2y = 2
y=1
With no substitution.

For substitution you solve 1) for x
x=5-y
then substitute in 2) to get
5(5-y)-3y=17
Then you solve that for y and substitute in 1) and solve for x.
 

Denis

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Feb 17, 2004
Messages
1,480
PreAlgebraDummy15 said:
ok now I'm doing substituting:
x+y=5
5x-3y=17
________
-4x-2y=-12
thats what I got so far (the bottom equation) if anyone can tell me if it's right and maybe help me with the next step that would be great! Thanks
WHY did you subtract ? Is that the way you were taught? I HOPE NOT!

Multiply 1st equation by 3; now you have:
3x + 3y = 15
5x - 3y = 17
Now add them:
8x = 32 : OK?

IF you were TOLD to use substitution, then this way:
x+y=5 : y = 5 - x
Substitute that in 2nd equation:
5x-3y=17 : 5x - 3(5 - x) = 17 : 5x - 15 + 3x = 17 : 8x = 32 ; OK?
 

PreAlgebraDummy15

New member
Joined
Nov 6, 2005
Messages
38
Substituting (More) Still urgent!

It did thanks. I have another question:

7c+5d=18
c-5d=-2
________
6c=16
__ __
6 6


c=2 4/6= 2 2/3

Is that right so far?
 

jolly

Junior Member
Joined
Apr 18, 2004
Messages
124
Re: Substituting (More) Still urgent!

PreAlgebraDummy15 said:
It did thanks. I have another question:

7c+5d=18
c-5d=-2
________
6c=16
__ __
6 6


c=2 4/6= 2 2/3

Is that right so far?
Why is everything always "urgent" with you? It gets old and annoying. FYI- New problems, new posts.
 

Gene

Senior Member
Joined
Oct 8, 2003
Messages
1,904
You seem to have problems with adding and subtracting. You are adding here. Why do you think that 7+1=6?????
And you still are not substituting. You are eliminating.
 
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