The Distance Formula
How far apart are the two points (2,0) and (5,0)? Without even drawing a graph, you might be able to say they are 3 units apart. They're both on the x axis, so it's just a straight line measurement. But how can you measure the distance between two random points on a graph? Use the distance formula:
At first glance, that formula looks like a real mess! But just think about the x2-x1 and y2-y1 as the length in each direction. Look at a geometric expression of the distance formula:
It doesn't matter which point is (x1,y1) and which is (x2,y2). The key idea to get from this plot is that you only care about the change in x and the change in y. You'll use each of those measurements as the side of a triangle, where the hypotenuse is the distance between the two points. You could express the distance formula like this:
How did we come up with that formula? It's simply the pythagorean theorem, which allows us to find the hypotenuse of a right triangle. In our case, the hypotenuse is the distance between the two points!
Let's look at a simple example now:
Example:
Find the distance between the two points (5,5) and (1,2) by using the distance formula.
Solution:
Rather than blindly plugging numbers into a formula, draw a graph so you know what's going on.
Those are your two points. Remember that the important values are the change in x and the change in y. That's equivalent to saying, what is x2-x1 and what is y2-y1.
Here, the change in x is 4 units, and the change in y is 3 units. You can even count the lines on the graph to make sure that's correct.
Once we knew that x2-x1 was 4 and y2-y1 was 3, we just inserted those numbers into the distance formula to solve.
You might be wondering what happens if you switch the points? Remember that we are dealing with distances, which are inherently positive. So, just use the positive distance between two points. If you look at the formula, you'll notice that x2-x1 and y2-y1 are squared, which automatically makes them positive anyway. So it doesn't matter which point is which -- you'll still get a positive distance!
Let's look at another example and solve it without using a graph:
Example:
Find the distance between the two points (8,-2) and (3, 9).
Solution:
Now that you understand how the distance formula works, you can plug the numbers straight into the formula:
The distance formula is not complicated -- you just need to practice with a graph so you understand what's going on. You're simply making a triangle and finding the length of the hypotenuse. The distance formula is just the pythagorean theorem!
What about a three-dimensional space? How would we find the distance between the points (1,5,0) and (2,0, 8)? It's certainly a lot harder to draw a graph and measure the distance! The three-dimensional distance formula is actually super simple. Just add another term into the formula we have to account for the z-axis:
If you've made it this far, hopefully you have a better understanding of the distance formula. If that's not the case, here's another distance formula lesson available online.
