Posted by **TyTy** on Saturday, January 5, 2008 at 5:34pm.

This is kind of like the first question I asked, and I'm still having a little trouble on it.

The Ordered pairs in each exercise are for the same direct variation.

Find the missing value.

(-2,8) and (x,12)

I'm not sure how to work out the problem.

- Stuck on Algebra 1 -
**TyTy**, Saturday, January 5, 2008 at 6:18pm
I'm not sure but if i cross multiply the two coordinates, i think the answer is...

x=-3?

- Stuck on Algebra 1 -
**Writeacher**, Saturday, January 5, 2008 at 6:33pm
Cross-multiplying seems to be the process:

http://www.jiskha.com/display.cgi?id=1199568727

So ...

-2/8 = x/12

8x = -24

x = -3

Right?

- Stuck on Algebra 1 -
**TyTy**, Saturday, January 5, 2008 at 6:46pm
yes, x=-3

- Stuck on Algebra 1 -
**Damon**, Saturday, January 5, 2008 at 6:54pm
Although I am sure I will be called on the carpet for saying why, I will try anyway.

If it is a direct variation it is a straight line of form:

y = k x

put in your first point

8 = -2 x

so k = -4

NOW any other point on this line satisfies

y = -4 x

so put in 12 for y

12 = -4 x

x = -3 as you know from "cross multiplying"

- Stuck on Algebra 1 -
**Damon**, Saturday, January 5, 2008 at 6:59pm
By the way, these problems are a special case of direct variation, linear.

The relation could be quadratic, cubic quartic ....

but is always of form y = k x^n

Here so far we are assuming that n = 1

- Stuck on Algebra 1 -
**Damon**, Saturday, January 5, 2008 at 7:00pm
Do you get it now?

- Stuck on Algebra 1 -
**Damon**, Saturday, January 5, 2008 at 7:03pm
Fixing typo and expanding

y = k x

put in your first point

8 = -2 k

solve that for k

so k = -4

NOW any other point on this line satisfies

y = -4 x

so put in 12 for y

12 = -4 x

x = -3 as you know from "cross multiplying"

- Stuck on Algebra 1 -
**Damon**, Saturday, January 5, 2008 at 7:41pm
I do not think you will get direct variation that is quadratic etc in this course or they would have mentioned it. For now assume direct variation means y = k x

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