The Distance Formula
The Distance Formula is a useful tool for finding the distance between two points. The formula itself is actually derived from the Pythagorean Theorem. That’s why we can claim that this formula is simply the Pythagorean Theorem in disguise.
If you want to see how the Distance Formula is derived from the Pythagorean Theorem, please check out my lesson on How to Derive the Distance Formula using the Pythagorean Theorem.
Distance Formula shown as the Derivative of the Pythagorean Theorem
The distance d between two points A (x1, y1) and B (x2, y2) is calculated as follows:
a) The expression x2 – x1 is read as “the change in x“.
b) The expression y2 – y1 is read as “the change in y“.
Examples of the Application of the Distance Formula
Example 1: Find the distance between the points (–3, 2) and (3, 5).
Label the parts of each point properly and substitute into the distance formula.
If we let (–3, 2) be the first point then it will take the subscript of 1, thus, x1 = –3 and y1 = 2.
Similarly, if (3, 5) be the second point it will have the subscript of 2, thus, x2 = 3 and y2 = 5
Here is the calculation,
This is how it looks on the graph.
Example 2: Find the distance between the points (–1, –1) and (4, –5).
If we assign (–1, –1) as our first point then
In the same manner, assigning (4, –5) as our second point, we have
Plugging the values of x and y, we get…
The two points and the distance between them look like this on the graph.
Example 3: Find the distance between the points (–4, –3) and (4, 3).
Sometimes you may wonder if switching the points in calculating the distance can affect the final outcome.
Well, if you think about it, the formula is squaring the difference of the corresponding x and y values. That means it doesn’t matter if the change in x or change in y is negative because when we eventually square it (raise to the 2nd power), the result always comes out to be positive.
Let’s “prove” that the answer is always the same by solving this problem two ways!
The first solution shows the usual way because we assign which point is the first and second based on the order in which they are given to us in the problem. In the second solution, we switch the points.
- Solution 1:
- Solution 2:
As you can see, both answers came out the same which is d = 10. Below is the visual solution to the problem.
Example 4: Find the radius of a circle with a diameter whose endpoints are (–7, 1) and (1, 3).
Remember that the diameter of a circle is twice the length of its radius. If that’s the case, then the radius is half the length of the diameter.
Here’s the plan! Since we are given with the endpoints of the diameter, we can use the distance formula to find its length. Finally, we divide it by 2 to get the length of the radius, as required by the problem.
- Find the length of the diameter with endpoints (–7, 1) and (1, 3).
- Solve for the radius by dividing the diameter by 2.
- The blue dots are the endpoints of diameter and the green dot is the center of the circle located at (-3,2).