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Posted by **Catherine** on Friday, January 25, 2008 at 9:27pm.

What are the coordinates of x and y? Use the Substitution Method.

---and---

-5x+7y= -41 and 7x+y=25

What are the coordinates of x and y? Use the Substitution Method.

Please help!!! =DDD

- Math -
**DrBob222**, Friday, January 25, 2008 at 10:18pmSubjects

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Homework Help Forum: Math

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Posted by Catherine on Friday, January 25, 2008 at 9:27pm.

12x+4y= 22 and 3x-8y= -10

What are the coordinates of x and y? Use the Substitution Method.**For the first one.**

12x+4y=22

3x-8y=-10

=============

I would solve equation 2 for x as

x=(-10+8y)/3

Then substitute this for x in equation 1 as

12[(-10+8y)/3] + 4y = 22

4(-10+8y) + 4y = 22

-40 + 32y + 4y = 22

36y = 40 + 22

36y = 62 + 40 = 62

y = 62/36 = 31/18

a strange number but it's right I think.

You substitute this value for y into equation 2 and solve for x. I found x to be 34/27, another strange number, but I think it's right. That's how you do these things. I'll leave the other one for you. Post specific questions if you get stuck. By the way, I substituted 31/18 for x and 34/27 for y and it checks. Check my work,especially for typos.

y = 30/36 =

---and---

-5x+7y= -41 and 7x+y=25

What are the coordinates of x and y? Use the Substitution Method

- Math -
**DrBob222**, Friday, January 25, 2008 at 10:26pmI don't know why my post copied all that extra trash but the part you want to read is in bold (except for the last y = 30/36 which has nothing to do with your problem). I'm pleased that Damon obtained the same strange numbers I did.

- Math -
- Math -
**Damon**, Friday, January 25, 2008 at 10:19pm12x+4y= 22 and 3x-8y= -10

These are both equations of lines and if they are not the same or parallel they will cross each other at one point, giving you a spot with x, y coordinates.

To solve it by substitution, solve either equation for either x or y and use that in the other equation. Then go back and find the value of the substituted variable with either original.

For example:

1.

solve the first equation for y

4 y = -12 x + 22

so

y = (-3 x + 11/2)

2.

Use that y in your second equation to get x

3 x - 8(-3x + 11/2) = -10

3 x + 24 x - 44 = -10

27 x = 34

x = 34/27

3.

use x = 34/27 in either original, like the first, to get y

12 (34/27) + 4 y = 22

136/9 + 4 y = 22

4 y = 62/9

y = 31/18

So, our intersection point is (34/27,32/18)

- Math -
**Damon**, Friday, January 25, 2008 at 10:45pmI checked the strange numbers back. They worked.

- Math -
**DrBob222**, Friday, January 25, 2008 at 11:19pmI did too because I didn't believe them (until I checked).

- Math -

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