### Video Transcript

In this video, we will use
properties of parallel lines and transversals to find the missing length of a line
segment in a transversal line cut by parallel lines. To do that, let’s see how parallel
lines can produce proportionality.

A property of parallel lines is
that if three or more parallel lines intersect two transversals, then they cut off
the transversals proportionally. Let’s consider what this might look
like. We have three parallel lines with
two transversals, both of which cross all three parallel lines. Remember that a transversal is a
line that crosses at least two other lines. In this figure, we’ll consider
these two transversals 𝑙 and 𝑚 since they are the transversals that cross all
three of the parallel lines. The three parallel lines cut by two
transversals in this figure creates four line segments.

Here, we have labeled those line
segments 𝑎, 𝑏, 𝑐, 𝑑. By this property, we can say that
the ratio 𝑎 to 𝑏 will be equal to the ratio of 𝑐 to 𝑑. Another way to write this
proportion is 𝑎 over 𝑐 will be equal to 𝑏 over 𝑑. Before we move on, it’s also worth
noting where we might see and use this property. This property is true and applies
inside of polygons. We can modify this figure to show
an example of this.

If we have the quadrilateral
𝐴𝐵𝐶𝐷, and it’s cut by the line segment 𝐸𝐹 that is parallel to 𝐴𝐷 and 𝐵𝐶,
notice that we have three parallel segments cut by two transversals. This means that the segments
created will be proportional. This means that segment 𝐴𝐸 over
segment 𝐸𝐵 will be equal to segment 𝐷𝐹 over segment 𝐹𝐶.

As an extension of this, there is
one more property we need to consider, congruent segments on transversals. If three or more parallel lines cut
off congruent segments on one transversal, then they cut off congruent segments on
every transversal. Here are three parallel lines. And if we have a transversal line
𝑙 such that the two created segments are congruent to one another, then any
transversal that is also cut by these three parallel lines will have congruent
segments. On the transversal 𝑚, the two
segments will be congruent to each other. And on the transversal 𝑛, the two
created segments are congruent to one another.

But we need to pay close attention
to this property. We’re saying that segment 𝑎 is
equal to segment 𝑏 or segment 𝑐 is equal to segment 𝑑. The congruency is on the same
transversal line, not between lines. You could not say here that segment
𝑎 was equal to segment 𝑐. Now we’re ready to take these
properties and apply them to some examples.

Using the information in the
figure, determine the length of line segment 𝐸𝐹.

First, we’ll identify line segment
𝐸𝐹. And then we’ll think about what we
already know based on the figure. We see in our figure that we have
three parallel lines. And so we can say that 𝐴𝐷 is
parallel to 𝐸𝐵, which is parallel to 𝐹𝐶. Then we can say that the lines 𝐷𝐹
and 𝐴𝐶 are transversals of the three parallel lines. Because we have three parallel
lines being cut by two transversals, we know that the created segments are cut
proportionally, by the properties of parallel lines and transversals.

This means we can say that 𝐷𝐸
over 𝐸𝐹 will be equal to 𝐴𝐵 over 𝐵𝐶. If we plug in the lengths we do
know, we have 48 over 𝐸𝐹 is equal to 47 over 141. To solve, we cross multiply. 48 times 141 will be equal to 47
times 𝐸𝐹. Therefore, 6768 is equal to 47
times 𝐸𝐹. And dividing both sides by 47 gives
us 144 is equal to 𝐸𝐹. Since our segments are measured in
centimeters, we can say that 𝐸𝐹 is equal to 144 centimeters.

In our next example, we’ll look at
a case where we have two transversals that are cut by four parallel lines.

In the figure, lines 𝐿 one, 𝐿
two, 𝐿 three, and 𝐿 four are parallel. Given that 𝑋𝑍 equals 12, 𝑍𝑁
equals eight, 𝐴𝐵 equals 10, and 𝐵𝐶 equals five, what is the length of segment
𝐶𝐷?

First, let’s take the information
we were given and use it to label our figure. We know that 𝐿 one, 𝐿 two, 𝐿
three, and 𝐿 four are parallel. We can add that to our figure. The line segment 𝑋𝑍 equals 12,
and 𝑍𝑁 equals eight. 𝐴𝐵 equals 10, and 𝐵𝐶 equals
five. The unknown length that we’re
trying to solve for is 𝐶𝐷. We can also note that the lines 𝑀
and 𝑀 prime are transversals of all four parallel lines. Because we have three or more
parallel lines that are cut by two transversals, we know that the created segments
on these transversals will be proportional. Therefore, by parallel lines in
transversal properties, we can set up a proportion to solve for the missing segment
𝐶𝐷.

𝑍𝑁 over 𝐶𝐷 is equal to 𝑋𝑍
over 𝐴𝐶. If we plug in what we know, we’ll
have eight over 𝐶𝐷 is equal to 12 over 15. To find the distance from 𝐴 to 𝐶,
we need to add 10 and five together, which gives us the 15. To solve, we cross multiply. Eight times 15 will be equal to 12
times 𝐶𝐷. 120 equals 12 times 𝐶𝐷. Dividing both sides by 12, we see
that 𝐶𝐷 will be equal to 10.

If we add this back to our figure,
we notice something interesting. The segment 𝐴𝐵 is equal to the
segment 𝐶𝐷. These are congruent segments. And because we know something about
congruent segments and transversals, this means we can also say that the segment
𝑋𝑌 is going to be congruent to the segment 𝑍𝑁. And therefore 𝑋𝑌 equals eight and
𝑌𝑍 must be equal to four. However, this question was only
asking for the length of the line segment 𝐶𝐷. And we’ve shown that 𝐶𝐷 equals 10
length units.

Let’s consider another example.

Given that 𝑋𝐿 equals nine
centimeters, find the length of line segment 𝑋𝑍.

First, let’s think about what we
see in this image. First of all, we have four parallel
lines: 𝐴𝑋, 𝐵𝑌, 𝐶𝑍, and 𝐷𝐿. We can also see that the lines 𝐴𝐷
and 𝑋𝐿 are transversals of the four parallel lines. We also see that on the line 𝐴𝐷,
the segments 𝐴𝐵, 𝐵𝐶, and 𝐶𝐷 are congruent to one another. And when three or more lines cut
congruent segments on one transversal, we know it will cut congruent segments on all
transversals cut by those parallel lines. This is the congruent segments on
transversals property, which means the line segment 𝑋𝑌 is congruent to the segment
𝑌𝑍, which is congruent to the segment 𝑍𝐿.

We also know that the distance from
𝑋 to 𝐿 equals nine centimeters. And since the segment 𝑋𝐿 is made
up of three congruent segments, we divide nine by three to find the length of each
of the segments. And then we see that 𝑋𝑌, 𝑌𝑍,
and 𝑍𝐿 will all measure three centimeters. We’re looking for the length of the
segment 𝑋𝑍, which is made up of two segments, 𝑋𝑌 and 𝑌𝑍, both of which measure
three centimeters. And so we can say that segment 𝑋𝑍
is equal to six centimeters.

In our final example, we have three
parallel lines cut by two transversals, and we’re solving for two unknown
values.

In the given figure, find the
values of 𝑥 and 𝑦.

First, we see that we have three
parallel lines, line 𝐽𝑀, 𝐾𝑃, and 𝐿𝑄. We can also say that the line 𝐽𝐿
and the line 𝑀𝑄 are transversals of the three parallel lines. Additionally, line segment 𝑀𝑃 and
line segment 𝑃𝑄 are congruent to one another. Based on these three things, we can
say that we have congruent segments on transversals. Since three parallel lines cut
congruent segments on one transversal, they will cut congruent segments on the other
transversal.

This means that line segment 𝐽𝐾
will be congruent to line segment 𝐾𝐿. And we can mark these congruent
segments on our figure. To find 𝑥 and 𝑦, we can then set
up two equations. We can say that 𝑀𝑃 is equal to
𝑃𝑄 and 𝐽𝐾 is equal to 𝐾𝐿. Starting with 𝑀𝑃 equals 𝑃𝑄, we
substitute five 𝑦 minus 25 in for 𝑀𝑃 and three 𝑦 minus seven in for 𝑃𝑄. We add 25 to both sides of our
equation, which will give us five 𝑦 is equal to three 𝑦 plus 18. So we subtract three 𝑦 from both
sides to get two 𝑦 equals 18. And finally, dividing both sides by
two gives us 𝑦 equals nine.

We’ll follow a similar procedure to
solve for 𝑥. We substitute six 𝑥 minus 20 in
for 𝐽𝐾 and four 𝑥 minus eight in for 𝐾𝐿. By adding 20 to both sides, we find
six 𝑥 equals four 𝑥 plus 12. From there, we subtract four 𝑥
from both sides, which gives us two 𝑥 equals 12. Dividing both sides by two, we find
that 𝑥 equals six. We could take the values we found
for 𝑥 and 𝑦 and use them to find the length of each line segment. 𝐽𝐾 would be equal to six times
six minus 20, which is 16. And because we know that 𝐾𝐿 is
congruent to 𝐽𝐾, it would also be equal to 16. And then 𝑀𝑃 would be equal to
five times nine minus 25, which is 20. And as 𝑃𝑄 is congruent to 𝑀𝑃,
both of these lengths would be 20. However, here we were only asked
what the values of 𝑥 and 𝑦 were. And we can say that 𝑥 equals six
and 𝑦 equals nine.

Before we finish, let’s quickly
review the key points from this video. If three or more parallel lines
intersect two transversals, then they cut the transversals proportionally. This property is also true for
parallel lines and transversals in polygons. And finally, if three or more
parallel lines cut congruent segments on one transversal, then they cut congruent
segments on every transversal.