Posted by **Angie** on Sunday, March 22, 2009 at 4:06pm.

Is it possible to use these points: (4,-2),(9,-8)

and write an equation in slope intercept form, without first writing an equation in point-slope form, and then rewriting it to slope-intercept form?

- Math -
**Reiny**, Sunday, March 22, 2009 at 4:18pm
sure, first find the slope

slope = (-8+2)/(9-4) = -6/5

then the equation is y = (-6/5)x + b

sub in (4,-2) , or the other point

-2 = (-6/5)(4) + b

I would now multiply by 5 (since I try to avoid fractions as much as possible)

-10 = -24 + 5b

b = 14/5

so y = (-6/5)x + 14/5

- Math -
**GanonTEK**, Sunday, March 22, 2009 at 4:19pm
No, you have to first use

y - y1 = m(x - x1) [point slope form]

and then rewrite in the form

y = mx + c [slope intercept form]

When you are given 2 points you have to find the slope first

[m = (y2 - y1)/(x2 - x1)]

then sub in the slope with either of the points in the equation in point slope form.

Hope that helps.

- Math -
**GanonTEK**, Sunday, March 22, 2009 at 4:21pm
I'll correct myself, yes you can I didn't think of the above method by Reiny.

I never used that way before :)

It's quite useful though.

Sorry for my error

- Math -
**Angie**, Sunday, March 22, 2009 at 5:34pm
why do you have to sub in (4,-2)? why can't you use 9,-8)?

- Math -
**Reiny**, Sunday, March 22, 2009 at 5:40pm
As I said, you can use either point.

I usually use the point that has the "nicer" numbers. Either point will give you the same value of b

A good idea is to then use the other point to see if the equation works.

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