Point-Slope Form to Standard Form Conversions

In this post, I’m going to explain a very frequently requested topic – how to convert your equation of a line from point-slope form to standard form. Sounds easy, right? Well, it isn’t difficult at all, provided that you understand the terminology and know what you’re doing. Follow along and hopefully all will become clear!

You are familiar with the general form of y=mx+b (also known as slope-intercept form), and you know that this equation tells you all that is necessary to actually graph this line – namely, the slope and y-intercept. However, what about if you have a section of your line up in quadrant I at the ordered pair coordinate of (150332, 23098)? The y-intercept on this graph doesn’t seem terribly useful way over here at this distant point! In this case, it’s probably more appropriate to use the point-slope form for your equation of a line. I need to explain this form first, before going on to show you how to convert from point-slope form to standard form equations.

In the most simple explanation that I know of, you can very easily derive the point-slope form from a very well known concept: the slope formula! Recall that slope is equal to rise over run. But what does that mean, in terms of mathematical symbols. Well, as I explained already in a previous post, this refers to the difference in the y values between two points, divided by the difference of the x values between those same two points. In formula form, you get something like this to define slope:

finding the slope

Slope equation

Now, to arrive at the point-slope form, all you need to do is a very simple rearrangement, as follows. Then, let the y2 and x2 just be x and y, and you are left with what you need to know:

point slope form

Point slope form

Hopefully, you can see the manipulation that I did there. I simply multiplied both sides by the denominator, and then switched sides so that you could see the more conventional form of this equation of a line. The 1’s and 2’s aren’t particularly important – here, the y1 and x1 terms are simply referring to a specific point, whereas x and y refers to any point.

To actually use this equation, you have a few ways. In one way, you can substitute in the m value and a given coordinate that is on your line for the y1 and x1 terms, and then go from there to simplify or solve for another point. Secondly, you can use two separate points to calculate the slope (remember, this is essentially just a rearranged slope formula!) Either way, this form of the equation of a line is incredibly useful and handy to know. And thankfully, being able to derive it easily from the slope formula gives you an easy way to come up with it if you can’t seem to remember it exactly when you need it the most (on exams!). :)

So, now that you know what point-slope is, let me refer you back to my previous post about standard form graphing equations – because, now I’m going to explain to you how to convert from point-slope to standard form. This isn’t a terribly complicated process, though it is extremely important to get right, because when done correctly, both forms mathematically represent the same line on a graph. Though, if you make an error, you will likely wind up with a different line altogether. It is important to pay attention to what form of the equation of a line you are being asked to provide, and then it’s just a matter of doing some of these steps!

Point-Slope Form to Standard Form

Example: Express the following equation in standard form, and state the values for A, B, and C.

point slope to standard form example

As a first point, I want you to realize that this example is very explicitly provided in point-slope form – to the letter! It won’t always be so! In any case, here is the basic strategy of what you want to do: get all of the x’s and y’s together on one side, and get the constants (i.e. no variables) over to the other side. Then, it’s just a matter of combining like terms and simplifying things wherever possible. Probably the most important thing to remember here is that you need to multiply what’s inside the brackets by the constant on the outside! This is far too easy a step to miss, but will completely mess you up!

point slope to standard form example steps

Hopefully you can follow along with those steps! All it is really is a rearrangement of the terms, grouping the x’s and y’s together, and the constants alone. When you get it into the final form as I have shown, it is easy to simply read off the values for A (the coefficient in front of the x), B (the coefficient in front of the y), and C (the constant with no variable attached to it). In this case, A is 2, B is -1, and C is -10. Remember, no number in front of the y means a 1 is assumed, and since the standard form has a +, in this case, the minus means there is a -1.

Try another one, a bit harder this time?

Example: Express the following equation in standard form, and state the values for A, B, and C.

point slope to standard form harder example

In this case, note that it isn’t immediately in point-slope form – I’ve reversed the left side terms. Of course, it’s a simple matter of just rearranging these, like so:

 point slope to standard form harder example step1

There, now that’s more appropriate. Next, we just follow the same steps that we did above: multiply through the brackets, and then group the x’s and y’s and isolate the constants. Easy, right? Let’s see what we get.

point slope to standard form harder example step2

I did all of the adding and subtracting on one line this time, but I did the same steps as I outline above, and as you can see, I have a final answer expressed in standard form! If you were to stop here, and say that A is 2/3, B is 1, and C is 7, you would most definitely be correct. However, there is a convention that many teachers and professors follow, and that is to remove everything from the denominator, wherever possible. In other words, teachers don’t like fractions! So, how do we get rid of our fraction? You probably have already figured out where I’m going with this – you simply have to multiply everything on both side by 3, the denominator, to cancel it out. Doing so, you wind up with this final standard form graphing equation:

point slope form to standard form answer

In this case, A is 2, y is 3, and C is 21. Another note – these coefficients are different from those we originally got, but the underlying math is all the same still. You can take both forms of our answers, create a table of values for each, and manually plot out the lines to prove that these indeed are the same lines, even though the equations look a bit different. You will probably agree that this version of the equation of the line just looks a lot nicer. :)

So, hopefully those few examples have properly explained to you the steps to consider when you have to convert point-slope form to standard form graphing equations. It’s not as difficult as it sounds, you just have to remember the points I’ve described in this post. In the next post, I’ll expand this concept to explain how slope-intercept form fits into all of this. Eventually, you won’t even recognize what form you are actually working with. You will just recognize what you need to do with the numbers to get the information that you need to solve your problem.

Thanks for reading this rather lengthy post! Please remember to subscribe or click on one of the Follow buttons on the right side of this page! I appreciate the support! And don’t forget, comments are always welcome if you need more explanations!

 


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