# How are the Pythagorean Theorem and the Distance Formula related?

##### 2 Answers

If we consider what the distance formula really tells you, we can see the similarities. It is more than just a similar form.

The **distance formula** is commonly seen as:

#D = sqrt((x_1 - x_2)^2 + (y_1 - y_2)^2)#

We commonly write the **Pythagorean Theorem** as:

#c = sqrt(a^2 + b^2)#

**Consider the following major points (in Euclidean geometry on a Cartesian coordinate axis):**

- The
*definition of a distance*from#x# to#pmc# is#color(green)(|x-c|)# . - There is the relationship where
#sqrt((x-c)^2) = color(green)(|x-c|) = x-c " AND " -x+c# - The distance from one point to another is the definition of a line segment.
- Any diagonal line segment has an
#x# component and a#y# component, due to the fact that a slope is#Deltay"/"Deltax# . The greater the#y# contribution, the steeper the slope. The greater the#x# contribution, the flatter the slope.

What do you see in these formulas? Have you ever tried drawing a triangle on a Cartesian coordinate system? If so, you should see that these are two formulas relating the diagonal distance on a right triangle that is composed of two component distances

Or, we could put it another way through substitutions based on the *distance definitions* above. Let:

#x_1 - x_2 = pma#

#y_1 - y_2 = pmb#

(depending on if#x_1 > x_2# or#x_1 < x_2# , and similarly for#y# .)

Now what do you see? An **equivalence**.

#D = color(blue)(sqrt((pma)^2 + (pmb)^2)) = c = color(blue)(sqrt(a^2 + b^2))#

*In short, the distance formula is a formalization of the Pythagorean Theorem using #x# and #y# coordinates. In other words, *

**they are the same thing in two seemingly different contexts.**

Essentially, they are "the same thing". As long as we stay in the context of Euclidean Geometry (not in the contemporary sense), the distance formula can be derived from the Pythagorean Theorem.

#### Explanation:

The **distance formula** makes sense in a coordinate context. It's used to compute the distance between two points in an orthogonal coordinate system (i.e. a plane with a coordinate system such that).

First of all, let's compute the distance between

This is quite easy, because the two points lie on the horizontal line

Similarly the distance between

Now we have all the ingredients we need. In fact, let's think about computing the distance between the points

So to compute the distance between

We got rid of the absolute values because squaring a number

Now, since

**NOTE:**

The contemporary way of thinking of distances is significantly different. Nowadays, the *definition* of distance between two points deeply influences the *geometry* of the space you work in, and the euclidean distance (the one that corresponds to Euclidean Geometry) is just one of the (infinitely-)many possible choices. So given - for example - a plane, one has first to define how the distance is computed and from this it will automatically follow the geometry of the plane (i.e. which lines can be called "straight", which are the triangles, ...) and if the Pythagorean Theorem (or some variation of it) holds. More technical details can be found in Wikipedia's article about enter link description here .

It may seem an abstract useless thing, but it's tremendously useful: Einstein's relativity and other important sector of physics are built on this. A curious and (hopefully) easy-to-understand example is the taxicab distance .