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Lesson 8.4: Distance Between Points

8.4 The Distance Between Points
A final application of the Pythagorean Theorem is on the coordinate plane. We can easily find the
distance between two points vertically or horizontally on a coordinate plane just by counting, but finding the
exact distance diagonally we have not been able to do until now.
The Distance between Any Two Points
On a coordinate plane, we can now find the distance between any two points by drawing in a right
triangle and using the Pythagorean Theorem. Consider the following example:
Notice that if we want to find the distance between these two points,
б€є2,2б€» and б€є5,6б€», we need to find the length of ЭЂ. Also note that Э„ is the
horizontal distance between the points and ‫ ݒ‬is the vertical distance between
the points. With all those values we now have a right triangle and can use the
Pythagorean Theorem as follows:
݄ଶ ൅ ‫ ݒ‬ଶ ൌ ݀ ଶ
3ଶ ൅ 4ଶ ൌ ݀ ଶ
9 ൅ 16 ൌ ݀ଶ
25 ൌ ݀ଶ
So we know that the distance between these points is five units. While this is easy to see when drawn out
on the coordinate plane, there are times when we are given the two points without a picture. In that case, we
have two options. We can either draw the points on the coordinate plane as above, or we can find the horizontal
and vertical distance between the points in another way.
To do this without graphing, we realize that the horizontal distance between two points is the difference
in their ‫ ݔ‬values. Why is this? Similarly, the vertical distance between two points is the difference in their ‫ݕ‬
values. Again, can you explain why?
So let’s look at our two points again, ሺ2,2ሻ and ሺ5,6ሻ. The horizontal distance would be the difference
between 2 and 5. Since difference means subtract, we can take 5 െ 2 ൌ 3 to find the horizontal distance is 3.
Similarly we can subtract the ‫ ݕ‬values to get 6 െ 2 ൌ 4 meaning a vertical distance of 4. We can then plug in 3
and 4 into the Pythagorean Theorem and solve exactly as above.
Enrichment: The Distance Formula
Using the information above, how would we find the distance between two generic points? We typically
represent generic points with the notation of ሺ‫ݔ‬ଵ , ‫ݕ‬ଵ ) and (‫ݔ‬ଶ , ‫ݕ‬ଶ ). So what would the horizontal and vertical
distance between these two points be?
Horizontal distance: ℎ = ‫ݔ‬ଶ − ‫ݔ‬ଵ
Vertical distance: ‫ݕ = ݒ‬ଶ − ‫ݕ‬ଵ
Finally, let’s substitute these into the Pythagorean Theorem of ℎଶ + ‫ ݒ‬ଶ = ݀ଶ as follows and then solve for
ЭЂ since ЭЂ is the actual distance between the points.
(‫ݔ‬ଶ − ‫ݔ‬ଵ )ଶ + (‫ݕ‬ଶ − ‫ݕ‬ଵ )ଶ = ݀ଶ
ඥ(‫ݔ‬ଶ − ‫ݔ‬ଵ )ଶ + (‫ݕ‬ଶ − ‫ݕ‬ଵ )ଶ = ඥ݀ଶ
ඥ(‫ݔ‬ଶ − ‫ݔ‬ଵ )ଶ + (‫ݕ‬ଶ − ‫ݕ‬ଵ )ଶ = ݀
The final result is what is known as the distance formula. Let’s use this formula to find the distance
between the points (в€’3, 4) and (3, в€’4).
݀ = ඥ(‫ݔ‬ଶ − ‫ݔ‬ଵ )ଶ + (‫ݕ‬ଶ − ‫ݕ‬ଵ )ଶ
݀ = ඥ(3 − (−3))ଶ + ((−4) − 4)ଶ
݀ = ඥ(6)ଶ + (−8)ଶ
ЭЂ = в€љ36 + 64
ЭЂ = в€љ100
ЭЂ = 10
We see that the distance between those two points is ten units. While the distance formula works, it is
often easier to simply visualize the horizontal and vertical distance between two points mentally or on a
coordinate plane. The distance formula is basically a fancy way to use the Pythagorean Formula and is meant for
enrichment only.
Lesson 8.4
Determine the distance between the given points. Round your answers to three decimal places if necessary.
1. б€є1, 3) and (4, 7)
(в€’3, 3) and (2, в€’9)
3. ሺെ2, −5) and (3, −8)
(в€’3, в€’3) and (3, 3)
5. б€є3, в€’2) and (5, 0)
(в€’3, в€’9) and (в€’3, 9)
7. б€є2, 1) and (3, в€’3)
(4, в€’2) and (7, 2)
9. б€є1, 1) and (7, 9)
10. (в€’8, 2) and (6, 2)
11. ሺെ4, 6) and (6, 2)
12. (2, 4) and (5, в€’2)
13. ሺെ5, −3) and (6, 6)
14. (в€’5, 4) and (7, 3)
15. ሺെ9, −3) and (−4, 4)
16. (2, в€’4) and (5, 4)
17. б€є0, 7) and (4, 2)
18. (в€’8, 7) and (7, в€’5)
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