Video Transcript
In this video, weโll learn what it
means for a shape to be a parallelogram. Weโll then discover the important
properties that a parallelogram has. We can use these properties to help
us find unknown angles or lengths.
Letโs begin with a mathematical
definition of a parallelogram. This will be a quadrilateral or
four-sided shape with both pairs of opposite sides parallel. So we could draw a parallelogram
like this or one like this or even like this. All they have to have is both pairs
of opposite sides parallel. You might wonder why weโve drawn a
rectangle here, but remember that a rectangle is just a special type of
parallelogram. It will still have both pairs of
opposite sides parallel. Squares and rhombuses would also be
two other special types of parallelogram.
Weโll now take a closer look at
some of the important properties. When weโre investigating the
properties of a parallelogram, we can just use a general parallelogram rather than a
special one, like a square or a rectangle. So here we have parallelogram
๐ด๐ต๐ถ๐ท. The first property of a
parallelogram is that opposite sides are parallel. And we know that from the very
definition of a parallelogram. The second property of
parallelograms are that opposite sides are equal or congruent. Letโs look at this parallelogram
and see if we can understand why this would be the case. We can split our parallelogram
along the diagonal ๐ด๐ถ. We can then think about the angles
that we would create.
Looking at angle ๐ต๐ด๐ถ because we
have two parallel lines and a transversal, then we also have an alternate angle here
at ๐ด๐ถ๐ท. So these two angles would be
equal. For the same reasons, we can say
that angle ๐ท๐ด๐ถ will be equal in size to angle ๐ต๐ถ๐ด, once again alternate
angles. Looking at the lengths, the line
๐ด๐ถ is common to both of these triangles. Donโt worry if you havenโt done too
much on the congruency of triangles. But weโve actually got enough
information here to prove that the two triangles ๐ท๐ด๐ถ and ๐ต๐ถ๐ด are congruent
using the angle-side-angle rules. We have two pairs of corresponding
angles congruent and a corresponding side congruent. When two triangles are congruent,
that means theyโre exactly the same shape and size.
In terms of helping us to prove the
property that opposite sides are equal, as we have two congruent sides, we could say
that this length ๐ต๐ด in triangle ๐ต๐ถ๐ด is congruent or equal to this length of
๐ท๐ถ in triangle ๐ท๐ด๐ถ. So these two sides of the
parallelogram are equal in length. In triangle ๐ต๐ถ๐ด, the length ๐ต๐ถ
will be congruent with the length ๐ท๐ด. And so weโve shown that the other
two sides are also equal in length, and so proving the second property.
The third property of
parallelograms is that opposite angles are equal. We can use the congruency of the
two triangles within it to demonstrate this. We could see how the sum of the
angles here at angle ๐ด would be equal to the sum of the angles here at angle
๐ถ. We could write that angle ๐ท๐ด๐ต is
equal to angle ๐ต๐ถ๐ท. In our congruent triangles, this
angle at ๐ด๐ท๐ถ will correspond with this angle at ๐ถ๐ต๐ด. So these two angles will also be
equal, and so proving the third property.
The next property of parallelograms
is that the sum of two adjacent angles is 180 degrees. We could alternatively write this
as adjacent angles are supplementary. What we mean by this is if, for
example, we took the angle ๐ด and ๐ต and added those together, they would add up to
180 degrees. Angle ๐ต and ๐ถ would also add to
180 degrees, and so would angle ๐ถ and ๐ท. We can prove this by remembering
that both pairs of opposite sides are parallel. Letโs look at angles ๐ต and ๐ถ.
We can recall that ๐ด๐ต and ๐ท๐ถ
are parallel and ๐ต๐ถ would be a transversal of these. We can recall that the sum of the
interior angles on the same side of a transversal is 180 degrees. And therefore, the angles ๐ต and ๐ถ
would add up to 180 degrees. If we look at angles ๐ถ and ๐ท
instead, then our two parallel lines here would be ๐ต๐ถ and ๐ด๐ท. The transversal would be the line
๐ถ๐ท. And so we can see again how the sum
of these two angles would add up to 180 degrees.
The final property weโre going to
look at in this video is that the diagonals of a parallelogram are bisectors. If theyโre bisectors, that means
they will cut each other exactly in half. So letโs have a look at why this
would happen. We can label this point where they
cross with the letter ๐ธ. Letโs have a look at this angle
๐ท๐ด๐ธ. We should be comfortable now with
recognizing that this angle ๐ด๐ถ๐ต will be congruent to it as we have our parallel
lines and a transversal. In the same way, this angle ๐ด๐ท๐ธ
would be equal to the angle ๐ธ๐ต๐ถ.
You might realize weโre working
towards another pair of congruent triangles. But weโd need to show that thereโs
a corresponding side congruent. And weโve already shown that the
property of parallelograms is that opposite sides are equal in length. So the length ๐ด๐ท will be equal to
the length ๐ต๐ถ. And so we can say that triangle
๐ด๐ท๐ธ is congruent to triangle ๐ถ๐ต๐ธ using the angle-side-angle rule. Therefore, the length ๐ด๐ธ in
triangle ๐ด๐ท๐ธ corresponds with the length ๐ถ๐ธ in triangle ๐ถ๐ต๐ธ. And so we can say that these two
lengths are equal, and weโve also shown that this diagonal ๐ด๐ถ has been
bisected.
If we look at this length ๐ท๐ธ, we
know that it corresponds to ๐ต๐ธ in triangle ๐ถ๐ต๐ธ. And therefore, these two lengths
are equal and show that the second diagonal has also been bisected. Itโs worthwhile making a note of
these properties of a parallelogram. Theyโre useful for exams and weโll
need them as we go through the following questions. Letโs have a look at our first
question.
Which of the following statements
must be true about a parallelogram? Option (A) it has four sides of
equal length. Option (B) it has four right
angles. Option (C) it has four congruent
sides. Option (D) it has exactly one pair
of parallel sides. Or option (E) it has exactly two
pairs of parallel sides.
In order to answer this question,
weโll need to remember what exactly a parallelogram is. Itโs defined as a quadrilateral or
four-sided shape, with both pairs of opposite sides parallel. We could therefore draw a range of
different parallelograms. The important thing is that, in
each one, it will have both pairs of opposite sides parallel. So letโs have a look at the
different statements weโre given.
Looking at option (A), which says
it has four sides of equal length, we can see that the parallelograms weโve drawn
definitely donโt have four sides of equal length. We couldโve drawn a square or even
a rhombus, and that would have four sides of equal length. But we canโt say that about every
single parallelogram. And so option (A) is incorrect.
Option (C) is phrased
differently. It has the word congruent here,
referring to the sides, which means that itโd say it would have four equal
sides. We can see that this would not be
the case. Option (B) says it has four right
angles. Well, we know that a square or a
rectangle does have four right angles, but it doesnโt apply to every single
parallelogram. Therefore, option (B) isnโt
correct. Option (D) says it has exactly one
pair of parallel sides. Well, we know from the definition
that both pairs of opposite sides have to be parallel, so option (D) is incorrect,
which leaves us with option (E). It has exactly two pairs of
parallel sides.
The definition of a parallelogram
tells us that both pairs or two pairs of the opposite sides will be parallel. So option (E) is our correct
answer. It has exactly two pairs of
parallel sides.
In our next question, weโll find
some unknown lengths.
Find the lengths of line segment
๐ถ๐ท and line segment ๐ท๐ด.
If we look at this shape ๐ด๐ต๐ถ๐ท,
we can see that itโs a parallelogram, but how do we know that for sure? Well, a parallelogram is a
quadrilateral with both pairs of opposite sides parallel. We can see from the line markings
that ๐ถ๐ต and ๐ด๐ท are parallel, and so are ๐ท๐ถ and ๐ด๐ต. The question asks us to find the
length of this line segment ๐ถ๐ท and the line segment of ๐ท๐ด. In order to do this, weโll need to
remember a key property of parallelograms, that is, that opposite sides are
equal. To find the length of ๐ถ๐ท then,
itโs going to be the same length as ๐ด๐ต, which is the opposite side. So thatโs 15 centimeters.
To find our next length of ๐ท๐ด, we
can look at the opposite side, which is ๐ต๐ถ, and thatโs 13 centimeters. We can answer the question then
with our two lengths, ๐ถ๐ท equals 15 centimeters and ๐ท๐ด equals 13 centimeters.
In the next question, weโll find an
unknown angle in a parallelogram.
Given that ๐ด๐ต๐ถ๐ท is a
parallelogram and the measure of angle ๐ถ equals 68 degrees, find the measure of
angle ๐ด.
In this question, weโre told that
we have a parallelogram. And we can in fact see that we do
have both pairs of opposite sides parallel. Weโre given this angle ๐ถ is 68
degrees. But in order to find the angle ๐ด,
weโll need to recall an important property of parallelograms. In a parallelogram, opposite angles
are equal. Once we know this, itโs very simple
to see that the angle ๐ด and angle ๐ถ are opposite angles, meaning that theyโre both
68 degrees. So we can give our answer that
angle ๐ด is 68 degrees.
Letโs have a look at another
question.
๐ด๐ต๐ถ๐ท is a parallelogram in
which the measure of angle ๐ต๐ธ๐ถ equals 79 degrees and the measure of angle ๐ธ๐ถ๐ต
equals 56 degrees. Determine the measure of angle
๐ธ๐ด๐ท.
Because weโre told that ๐ด๐ต๐ถ๐ท is
a parallelogram, this means we can say that the line ๐ท๐ถ is parallel to the line
๐ด๐ต and the line ๐ด๐ท is parallel to the line ๐ต๐ถ. Weโre given the two angle
measurements of ๐ต๐ธ๐ถ and ๐ธ๐ถ๐ต, and weโre asked to find this angle ๐ธ๐ด๐ท. In order to work out this angle
measurement, weโll need to remember some of the properties of the angles in
parallelograms.
Firstly, we can remember that
opposite angles are equal, and we could also remember that the sum of two adjacent
angles is 180 degrees. In this parallelogram, the angle
thatโs opposite to ๐ด will be the angle ๐ถ. However, we donโt know this total
angle ๐ท๐ถ๐ต. We only know that ๐ธ๐ถ๐ต is 56
degrees. If we look at our second property,
if we found the angle measurement of ๐ต, then that would help us to work out ๐ด. So how can we find the measurement
of angle ๐ต?
As well as being part of a
parallelogram, this angle at ๐ต is also part of a triangle. We know that the angles in a
triangle add up to 180 degrees. So this angle of ๐ถ๐ต๐ธ is equal to
180 degrees subtract 79 degrees subtract 56 degrees. 180 subtract 79 gives us 101
degrees, and subtracting 56 from that gives us 45 degrees. Remember that this is not our
answer as we still need to work out the angle ๐ธ๐ด๐ท. Using the property that the sum of
two adjacent angles is 180 degrees, then we calculate 180 subtract 45 degrees,
giving us our answer of 135 degrees.
Letโs look at one final
question.
In the figure, ๐ด๐ต๐ถ๐ท and
๐ถ๐ต๐ป๐ are parallelograms. Find the measure of obtuse angle
๐ด๐ต๐ป.
Because weโre told that ๐ด๐ต๐ถ๐ท
and ๐ถ๐ต๐ป๐ are parallelograms, that means in ๐ด๐ต๐ถ๐ท, the line ๐ด๐ท and the line
๐ถ๐ต are parallel and ๐ถ๐ต and ๐๐ป will also be parallel. ๐ถ๐ท and ๐ด๐ต are parallel. And in the lower parallelogram, we
know that ๐ถ๐ and ๐ต๐ป are parallel. Knowing the properties of a
parallelogram will help us to find our unknown angle. Weโre asked to find ๐ด๐ต๐ป, the
obtuse angle, which means that itโs this angle marked in orange.
In order to find this obtuse angle,
weโll begin by seeing if we can find this reflex angle ๐ด๐ต๐ป. Letโs consider the parts of this
angle in each parallelogram, and weโll begin with trying to find this angle at
๐ด๐ต๐ถ. We should remember that in a
parallelogram, adjacent angles are supplementary. We have got two adjacent angles to
this angle at ๐ต. Weโve got this angle at ๐ด and the
angle at ๐ถ. Either of these would be
supplementary. However, as weโre actually given
the measure of this angle ๐ท๐ด๐ต, letโs use this angle to help us find our
unknown. The angle ๐ด๐ต๐ถ can be found by
calculating 180 degrees subtract 72 degrees, which gives us 108 degrees.
Now letโs see if we can find this
angle ๐ถ๐ต๐ป, and once again weโll use the fact that adjacent angles add up to 180
degrees. So we subtract 51 degrees from 180
degrees. And we can remember that 180
subtract 50 gives us 130, and subtracting another one would give us 129 degrees. Now, when we look at the diagram,
at point ๐ต, weโve got an angle of 108 degrees, an angle of 129 degrees, and we want
to find the remaining portion of this angle. Weโll need to remember that the
angles about a point add up to 360 degrees. Weโll need to calculate 360 degrees
subtract 129 degrees subtract 108 degrees. We can therefore give our answer
that the obtuse angle ๐ด๐ต๐ป is 123 degrees.
Weโll now summarize what weโve
learned in this video. We began with the definition of a
parallelogram that itโs a quadrilateral with both pairs of opposite sides
parallel. We can draw many different types of
parallelogram, and they even include squares, rectangles, and rhombuses. We saw some important properties of
parallelograms. By definition, opposite sides are
parallel, but we also saw how opposite sides are equal in length.
We saw two angle properties of
parallelograms. Firstly, opposite angles are equal,
and secondly the sum of any two adjacent angles is 180 degrees. Finally, we saw that the diagonals
of a parallelogram are bisectors. As we see in many geometry
problems, we also need to recall key facts, for example, the angles in a triangle
add up to 180 degrees or the angles about a point sum to 360 degrees. Learning the properties of
parallelograms will help us with these specific types of problems, but also with
many more.