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Connecting Algebra and Geometry Through Coordinates

Position of Points on a Line Segment

A point on a directed line segment partitions the segment in a specific ratio. In this lesson, given the segment's endpoints and ratio, the point's coordinates will be identified.

Catch-Up and Review

Here are a few recommended readings before getting started with this lesson.

Try your knowledge on these topics.
a Consider the given right triangles, with two parallel legs.
Right Triangle
Given that vertices A, B, and C are collinear and BE is parallel to CD, choose the correct similarity theorem that proves AEBBDC.
b In the following diagram, ABE is similar to ACD.
Scale Factor
Given that AB=5, AE=4, and CD=6, identify the scale factor of ABE to ACD and find the length of AC.
c Given two right triangles, ABC is the dilation of ADE by a scale factor of
Dilation
If the hypotenuse of ADE is 4, find the hypotenuse of ABC.

Challenge

Investigating a Partitioned Line Segment

Dylan's home is 4.5 miles away from Big Apple Circus. Dylan walks from his home to the circus to watch the flying trapeze performance.
Point on a Directed Line Segment
What are the coordinates of the point at which Dylan would have walked 3 miles?

Example

Partitioning a Directed Line Segment

In a math test, Emily is asked to find the position of point M that partitions the directed line segment KL in the ratio 3:2.

Directed Line Segment
Help her identify the coordinates of M.

Hint

Draw a right triangle with hypotenuse KM and another right triangle with hypotenuse KL.

Solution

The position of a point that partitions a directed line segment in a given ratio can be found by using a dilation. In this case, two right triangles can be drawn to identify the dilation. According to the given ratio, KL is divided into 3 and 2 parts, represented by KM and ML respectively, out of 5 parts in total.

Directed Line Segment
Notice that KNM and KPL are both right angles, and therefore they are congruent angles. Additionally, by the Reflexive Property of Congruence, it can be said that K is congruent to itself.
Therefore, by the Angle-Angle Similarity Theorem, it can be concluded that KMN and KLP are similar triangles. Furthermore, since KM represents of the length of the directed line segment KL, it can be said that KMN is a dilation of KLP by a scale factor k of
By the composition of the triangles, points M and N are on the same vertical line segment. Therefore, by finding the x-coordinate of N, the x-coordinate of point M can be identified. As derived from the diagram, the length of KP is 8 units. Therefore, KN can be found by multiplying 8 by the scale factor k.
Thereby, point N is located 4.8 units away from point K in the positive horizontal direction. As a result, the x- coordinate of point N is -4+4.8=0.8. This means that N is located at (0.8,1).
Directed Line Segment
Consequently, the x-coordinate of point M is also 0.8. To find its y-coordinate, MN should be found. In the diagram, it can be seen that the length of LP is 4 units. With this information, MN can be found by multiplying 4 by the scale factor.
Point M is located 2.4 units away from point N in the positive vertical direction. Since the y-coordinate of N is 1 and 1+2.4 is equal to 3.4, point M is located at (0.8,3.4).
Directed Line Segment

Discussion

General Method for Partitioning a Directed Line Segment

The approach used to find the coordinates of point M can be generalized algebraically. Suppose that M divides the directed line segment from A(x1,y1) to B(x2,y2) in the ratio a:b.
Point on a Directed Line Segment
Since M divides AB from A to B, point A is said to be the center of dilation. Next, two right triangles will be considered to show the dilation. The directed distances from A to B in the x- and y-directions can be identified by subtracting the endpoints of the segment.
Point on a Directed Line Segment
Since ADM and ACB are right angles, they are congruent to each other. Furthermore, by the Reflexive Property of Congruence, it can be stated A is congruent to itself. With this information, by the Angle-Angle Similarity Theorem, AMD and ABC are similar triangles.
The next step to identify the position of M is to express the ratio a:b as the scale factor of the dilation. Since the hypotenuses of AMD and ABC are a and a+b, respectively, the scale factor k is the quotient between a and a+b.
Because AMD and ABC are similar triangles, AD and MD can be found by multiplying AC and BC, respectively, by the scale factor k.
Finally, the coordinates of M can be found by adding the resulting distances to the coordinates of the center of dilation.



Example

Finding Points on a Triangle's Perimeter

In PQR, point S partitions PQ from point P to point Q in the ratio 3:7. Similarly, point T partitions RP from point R to point P in the same ratio. With this information, Heichi is trying to find the positions of points S and T.

Triangle

Given the endpoints of the directed line segments, help Heichi determine the coordinates of S and T.

Hint

Use the formula for identifying the position of a point on a directed line segment.

Solution

First, the given ratio will be expressed on PQ and RP.

Triangle
Next, the scale factor k can be found by using the formula for the scale factor of a dilation.
Now, the coordinates of S can be found by substituting the coordinates of the endpoints of PQ into the formula for the point that partitions the directed line segment. Note that S divides PQ from point P to point Q. Therefore, P(-2,5) and Q(-4,1) will be substituted for (x1,y1) and (x2,y2), respectively.
Evaluate
S(-2.6,3.8)
The coordinates of S were found. By following the same procedure, the coordinates of T can also be found.
Formula:
Segment Endpoints Scale Factor Substitute Simplify
PQ P(-2,5) and Q(-4,1) 0.3 S(-2.6,3.8)
RP R(5,1) and P(-2,5) 0.3 T(2.9,2.2)

Example

Partitioning Distances Between Cities on a Map

Vincenzo and Magdalena are two high school students. They are visiting different cities during their summer holidays. Vincenzo is currently traveling from Madrid to Warsaw, while Magdalena is traveling from Warsaw to Madrid.

Departure Time


The approximated coordinates of the cities are given in the following table.

City Longitude (x) Latitude (y)
Madrid
Warsaw

The distance between the cities is approximately 2000 kilometers. Give the coordinates of the points at which they would have flown 500 kilometers. Round the coordinates to the nearest hundredth.

Hint

Use the formula for identifying the position of a point on a directed line segment.

Solution

The coordinates of points V and M can be found by using the formula for identifying the position of a point on a directed line segment. The distance between Madrid and Warsaw is 2000 kilometers. It is given that each passenger has flown 500 kilometers. With this information, the scale factor of the dilation can be identified.
The scale factor for both routes is 0.25. From here, the coordinates of V can be found by substituting the coordinates of the cities and the scale factor into the formula. Since Vincenzo is traveling from Madrid to Warsaw, (-3.70,40.42) and (21.01,52.23) will be substituted for (x1,y1) and (x2,y2), respectively.
Evaluate
V(-3.70+6.1775,40.42+2.9525)
V(2.4775,43.3725)
V(2.48,43.37)
By following the same procedure, the coordinates of M can also be found. Be aware that Magdalena is traveling in the opposite direction of Vincenzo, from Warsaw to Madrid, while Vincenzo is traveling from Madrid to Warsaw.
Formula:
Route Endpoints Scale Factor Substitute Simplify
Madrid to Warsaw (-3.70,40.42) and (21.01,52.23) 0.25 V(2.48,43.37)
Warsaw to Madrid (21.01,52.23) and (-3.70,40.42) 0.25 M(14.83,49.28)

Even if Vincenzo and Magdalena travel between the same cities, they are in different positions. The reason for their differing positions is because they are traveling in opposite directions. This reasoning would suggest, a point's position on a directed line segment depends on which endpoint is used as the center of dilation.

Example

Partitioning a Distance Into Congruent Segments

A railroad company is planning to construct a railroad from Houston to Washington D.C. They will place four train stations between the initial and the final station. Each station will be equidistant to each other, as the diagram below shows.

Choo Choo

The coordinates of the cities are given in a table.

City Longitude (x) Latitude (y)
Houston
Washington D.C.

Find the coordinates of each station. Round them to the nearest hundredth.

Hint

Use the formula for identifying the position of a point on a directed line segment.

Solution

Consider station A. Because each station is equidistant — the same distance to eachother — station A partitions the railroad from Houston to Washington in the ratio 1:4. By using this information, the scale factor of the dilation can be found.
Now, by substituting the scale factor and the coordinates of the cities into the formula, the coordinates of station A can be found.
Evaluate
A(-95.37+3.666,29.76+1.83)
A(-91.704,31.59)
A(-91.70,31.59)
By following the same procedure, the coordinates of the other stations can also be found. First, the scale factors should be identified for each station.
Station Ratio Scale Factor Simplify
A 1:4 k=0.2
B 2:3 k=0.4
C 3:2 k=0.6
D 4:1 k=0.8

From here, the positions of the other stations can be identified. Keep in mind that all stations go from Houston to Washington. Therefore, the endpoints are always (-95.37,29.76) and (-77.04,38.91).

Formula:
Station Scale Factor Substitute Simplify
A 0.2 A(-91.70,31.59)
B 0.4 B(-88.04,33.42)
C 0.6 C(-84.37,35.25)
D 0.8 D(-80.71,37.08)

Closure

Finding a Point on a Directed Line Segment

By using the formula learned in this lesson, the challenge presented at the beginning can be solved. Recall that the coordinates of the point at which Dylan would have walked 3 miles of the 4.5-mile distance were asked to be found.
Point on a Directed Line Segment

Hint

Determine the scale factor of the dilation. Use the formula for identifying the position of a point on a directed line segment.

Solution

Notice that the point at which Dylan stands after he walked 3 miles partitions the road in the ratio 2:1.
With this information, the scale factor of the dilation can be determined.
By substituting the scale factor and the coordinates of the endpoints into the corresponding formula, Dylan's position can be found. Since he walks from his house to the circus, (2,5) and (6.5,2) will be substituted for (x1,y1) and (x2,y2), respectively.
Evaluate
D(2+3,52)
D(5,3)
Therefore, the coordinates of the point at which Dylan would have walked 3 miles are (5,3).
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