Simultaneous Equations - Substitution Watch

Coerce
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#1
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Hey,

Well I'm not very clear on this.. I know that x or y is needed to be made the subject. And then substituting the other equation in

the two equations are;

x+3y=11
4x-7y=6

So would I make X the subject (as its the easier one)

x=11 - 3y
Then how would I go about adding the second equation
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Mr M
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(Original post by Coerce)
Hey,

Well I'm not very clear on this.. I know that x or y is needed to be made the subject. And then substituting the other equation in

the two equations are;

x+3y=11
4x-7y=6

So would I make X the subject (as its the easier one)

x=11 - 3y
Then how would I go about adding the second equation
You don't add the second equation if you are trying to solve by substitution. Substitute into the second equation to obtain 4(11-3y)-7y=6 and then solve the linear equation to find y.
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Coerce
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(Original post by Mr M)
You don't add the second equation if you are trying to solve by substitution. Substitute into the second equation to obtain 4(11-3y)-7y=6 and then solve the linear equation to find y.
Oh that is what I meant.. but surely you couldn't work it out from 44-12y-7y=6?

so would it just be 5y = 60?
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Mr M
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(Original post by Coerce)
Oh that is what I meant.. but surely you couldn't work it out from 44-12y-7y=6?

so would it just be 5y = 60?
No!

44-12y-7y=6

Collect like terms:

44-19y=6

Subtract 6 from each side:

38-19y=0

Add 19y to each side:

19y=38

and so on.
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Coerce
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(Original post by Mr M)
No!

44-12y-7y=6

Collect like terms:

44-19y=6

Subtract 6 from each side:

38-19y=0

Add 19y to each side:

19y=38

and so on.

Ah I see LOL.. **** man I make myself look like a fool on her 80% of times-.- ty
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Coerce
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(Original post by Mr M)
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How would you go about 3x-y=7 and 10x +3y = -2
think the low numbers / minus answer is confusing me a bit
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TenOfThem
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(Original post by Coerce)
How would you go about 3x-y=7 and 10x +3y = -2
think the low numbers / minus answer is confusing me a bit
Just apply the same method
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Mr M
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(Original post by Coerce)
How would you go about 3x-y=7 and 10x +3y = -2
think the low numbers / minus answer is confusing me a bit
y=3x-7
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Coerce
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(Original post by Mr M)
y=3x-7
Yeah was double checking - you just substitute that into the other formula thanks both of you.. much appreciated
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moh'd seif
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substituting your new eq into
4x-7y=6 you will got
4(11-3y)-7y=6 then it wil become
44-12y-7y=6
-19y=6-44
-19y=-38 then dividing by -19 both side
So you will get y=2 and then take equation x+3y=11 by substituting the value of y then you wll got x=5
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TenOfThem
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(Original post by moh'd seif)
...
Even though the OP had solved this particular problem - others may use the thread to get support

Therefore, the rules asking us to not post full solutions will still apply
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