Tuesday

July 7, 2015

July 7, 2015

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

Art

Business

Computers

English

Foreign Languages

Health

Home Economics

Mathematics

Music

Physical Education

Science

Social Studies

Features

Contribute

Download

Experts

Forum

Job Openings

Homework Tips

Libraries.Jiskha

Link to Us

Reference

School Supplies

Search

Homework Help Forum: Math

Current Questions | Post a New Question | Answer this Question | Further Reading

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 -