View Full Version : Solve the system using the substittion method

bcroes

06-24-2009, 10:32 PM

Normally these don't get me but I'm stumped.

y=2x=9

y=7x=10

I just end up with tons of fractions and a screwed up solution.

The answer I'm looking for is (-1/5, 43/5) I just don't know how to get there.

Thanks in advance!

Aladdin

06-24-2009, 11:03 PM

Please show us your work, so that we know where to begin to help you

bcroes

06-24-2009, 11:21 PM

Sorry about that...

y = 2x + 9

y = 7x + 10

I know that I need to solve one equation for X so I'll choose the second

y = 7x + 10

-7x + y = 10

-7x = -y +10

x = 1/7y + 10/7

This is where I get confused (that is if the above is even correct)

Taking the first equation I enter 1/7y + 10/7 for X.

y = 2(1/7y + 10/7) + 9

y = 3/7y + 20/7 + 9

y = 3/7y + 83/7

4/7y = 83/7

y = 83/7 * 7/4

y = 581/28

Which can't be correct and is a total disaster. Haha please help me.

Aladdin

06-24-2009, 11:49 PM

Sorry about that...

y = 2x + 9

y = 7x + 10

I know that I need to solve one equation for X so I'll choose the second----> Why,

y = 7x + 10

-7x + y = 10

-7x = -y +10

x = 1/7y + 10/7

This is where I get confused (that is if the above is even correct)

Taking the first equation I enter 1/7y + 10/7 for X.

y = 2(1/7y + 10/7) + 9

y = 3/7y + 20/7 + 9

y = 3/7y + 83/7

4/7y = 83/7

y = 83/7 * 7/4

y = 581/28

Which can't be correct and is a total disaster. Haha please help me.

We may find the value of y in the first equation, which is given y=2x+9 & replace it in the second equation, which gives:

2x+9=7x+10

2x-7x=10-9

-5x=1

Consequently,x=-1/5

For x=-1/5

Then the solution of the system is (-1/5,43/5)

bcroes

06-25-2009, 12:34 AM

That is great! Once again a case of me over analyzing things.

Thank You!

Denis

06-25-2009, 09:24 AM

Paralysis by analysis :wink:

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