Using point-slope form, find the equation of the following points: (2,3) and (5,1)

## How to use point-slope form when you are given two sets of points?

# 2 Answers

The point slope form is written like this:

y-y_{1}=m(x-x_{1}) where m is the slope, and (x_{1},y_{1}) can be either one of the two points you are given. First, we need to find the slope m from the given points.

The slope m=rise/run=(y_{2}-y_{1})/(x_{2}-x_{1}). Let (2,3)=(x_{1},y_{1}) and (5,1)=(x_{2},y_{2}). Then the slope is:

m=(1-3)/(5-2)=-2/3

Now put the point (2,3) in the point-slope equation along with the slope m to find the equation of the line between the two points.

y-3=(-2/3)(x-2) Now distribute -2/3 through the parentheses, and add 3 to both sides

y=(-2/3)x+(4/3)+3

y=(-2/3)x+13/3 The term 13/3 is (4/3)+3 written in fraction form. This is the equation of the line in a form called slope-intercept form. In slope-intercept form, the slope is still -2/3, and the place where the line intercepts the y-axis is 13/3.

Hope that helps a bit!

Whitney, whenever you are wanting the equation for a straight line, you need to find two things: How steep the line is (the slope) and its height when x=0 (the y-intercept). In the end, there a few ways to represent this information, but a typical answer will look something like y = mx + b.

Point-slope form is one approach for finding a line and it requires two things also: A point and a slope. Pretty sweet name, huh?

1. A point. It looks like they gave you two to pick from here. Pick either (2,3) or (5,1). It doesn't matter which; the answer will come out the same.

2. A slope. Well, this question doesn't tell you the slope, so we'll need to calculate it. Slope is calculated by the "rise-over-run" equation. The rise is how much the y-coordinates changed, the run is how much the x-coordinates changed. Looking at
(2,3) and (5,1), how much did y change? It went from 3 to 1, so it changed by -2 (it went down 2). How much did x change? It went from 2 to 5, so it changed by 3 (it went up 3). **The slope is the change in y (rise) divided by the change in x (run)**.
The previous post has already done the arithmetic for you, but I'm sure you can do that on your own, too!

3. Plug-n-chug. Point-slope form looks like this:

**y - y _{1} = m(x - x_{1})**

The y_{1} and x_{1} are where you put the point that you picked in step one (remember that either point will be fine). The m is the slope that you calculated in step 2.

**At this point you are done!** Awesome! You have it in point-slope form, which is what the question asked you to do.

**Extra Credit Reading**

4. Slope-intercept form

But, could we do better? Probably. Read the assignment heading carefully and see whether it asks you to convert your answer to slope-intercept form.

If it does, then there is one more simple step. Slope-Intercept form is just another way to express the details of a line (kind of like having more than one word that means "awesome"). You need two things to use this form (you guessed it! A slope and an intercept). Right now, you have something that looks like this:

y - y_{1} = m(x - x_{1})

All you want to do is have it look like this instead:

y = mx + b

Do you see that they both have an m? That means that number will stay the same. So will y and x for the same reason. To get b, all you do is take your point and slope and use this equation:

b = y_{1} - mx_{1}

Fill in the x_{1} and y_{1} with the point you picked in step 1 and m with the slope you calculated in step 2. Do the arithmetic, and this will give you the value of b.

5. It really doesn't matter which point I pick?

Nope! Try both, and you'll see that **they both have the same slope-intercept form**.

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