# Math

posted by .

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 -

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 -

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 -

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 -

why do you have to sub in (4,-2)? why can't you use 9,-8)?

• Math -

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.