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Here are a few recommended readings before getting started with this lesson.
A conclusion that can be made from the previous exploration is that the opposite sides of a parallelogram are congruent. This is explained in detail in the following theorem.
The opposite sides of a parallelogram are congruent.
In respects to the characteristics of the diagram, the following statement holds true.
PQ≅SRandQR≅PS
This theorem can also be proven by using congruent triangles. Consider the parallelogram PQRS and its diagonal PR.
PQ≅SRandQR≅PS
Furthermore, it can be stated whether a quadrilateral is a parallelogram just by checking if its opposite sides are congruent.
If the opposite sides of a quadrilateral are congruent, then the polygon is a parallelogram.
Following the above diagram, the statement below holds true.
If PQ≅SR and QR≅PS, then PQRS is a parallelogram.
This theorem can be proven by using congruent triangles. Consider the quadrilateral PQRS, whose opposite sides are congruent, and its diagonal PR. By the Reflexive Property of Congruence, this diagonal is congruent to itself.
Finally, by the Converse of the Alternate Interior Angles Theorem, PQ is parallel to RS and QR is parallel to SP. Therefore, by the definition of a parallelogram, PQRS is a parallelogram.
This proves the theorem.
If PQ≅SR and QR≅PS, then PQRS is a parallelogram.
In a parallelogram, the opposite angles are congruent.
For the parallelogram PQRS, the following statement holds true.
∠Q≅∠Sand∠P≅∠R
This theorem can be proved by using congruent triangles. Consider the parallelogram PQRS and its diagonal PR.
Opposite sides of a parallelogram are parallel. Therefore, by the Alternate Interior Angles Theorem it can be stated that ∠QPR≅∠SRP and ∠QRP≅∠SPR. Furthermore, by the Reflexive Property of Congruence, PR is congruent to itself.
By drawing the diagonal QS and using a similar procedure, it can be shown that ∠P and ∠R are also congruent angles.
∠Q≅∠Sand∠P≅∠R
Furthermore, it can be determined whether a quadrilateral is a parallelogram just by looking at its opposite angles.
If both pairs of opposite angles of a quadrilateral are congruent, then the quadrilateral is a parallelogram.
Based on the above diagram, the following statement holds true.
If ∠A≅∠C and ∠B≅∠D, then ABCD is a parallelogram.
Assume that ABCD is a quadrilateral with opposite congruent angles. It should be noted that congruent angles have the same measure. Then, let x∘ be the measure of ∠A and ∠C, and y∘ be the measure of ∠B and ∠D.
To be able to be carefree and enjoy a soccer match over the weekend, Vincenzo wants to complete his Geometry homework immediately after school. He is given a diagram showing a parallelogram, and asked to find the values of a, b, and x.
First, for simplicity, the value of x will be found. After that, the values of a and b will be calculated.
(II): LHS+10=RHS+10
(I): a=3b
(I): Multiply
(I): LHS−10=RHS−10
(I): LHS−10b=RHS−10b
(I): LHS/5=RHS/5
(II): b=10
(II): Multiply
A conclusion that can be made from the previous exploration is that the diagonals of a parallelogram intersect at their midpoint. This is explained in detail in the following theorem.
In a parallelogram, the diagonals bisect each other.
If PQRS is a parallelogram, then the following statement holds true.
PM≅RMandQM≅SM
This theorem can be proven by using congruent triangles. Consider the parallelogram PQRS and its diagonals PR and QS. Let M be the point intersection of the diagonals.
Since PQ and SR are parallel, by the Alternate Interior Angles Theorem it can be stated that ∠QPR≅∠SRP and that ∠PQS≅∠RSQ. Furthermore, by the Parallelogram Opposite Sides Theorem it can be said that PQ≅SR.
PM≅RMandQM≅SM
By the definition of a segment bisector, both segments PR and QS are bisected at point M. Therefore, it has been proven that the diagonals of a parallelogram bisect each other.
Also, a quadrilateral can be identified as a parallelogram just by looking at its diagonals.
If the diagonals of a quadrilateral bisect each other, then the quadrilateral is a parallelogram.
Based on the diagram above, the following relation holds true.
If AC and BD bisect each other, then ABCD is a parallelogram.
Let E be point of intersection of the diagonals of a quadrilateral. Since the diagonals bisect each other, E is the midpoint of each diagonal.
Because ∠AEB and ∠CED are vertical angles, they are congruent by the Vertical Angles Theorem. Therefore, by the Side-Angle-Side Congruence Theorem, △AEB and △CED are congruent triangles. Since corresponding parts of congruent figures are congruent, AB and CD are congruent.
Applying a similar reasoning, it can be concluded that △AED and △CEB are congruent triangles. Consequently, AD and BC are also congruent.
Finally, since both pairs of opposite sides of quadrilateral ABCD are congruent, the Converse Parallelogram Opposite Sides Theorem states that ABCD is a parallelogram.
Vincenzo has one last exercise to finish before going to a soccer match. He has been given a diagram showing a parallelogram. He is asked to find the value of x and y.
According to the Parallelogram Diagonals Theorem, the diagonals of a parallelogram bisect each other.
It can be determined whether a parallelogram is a rectangle just by looking at its diagonals. Furthermore, if a parallelogram is a rectangle, a statement about its diagonals can be made.
A parallelogram is a rectangle if and only if its diagonals are congruent.
Based on the diagram, the following relation holds true.
PQRS is a rectangle ⇔ PR≅QS
Two proofs will be provided for this theorem. Each proof will consist of two parts.
This proof will use similar triangles to prove the theorem.
Suppose PQRS is a rectangle and PR and QS are its diagonals. By the Parallelogram Opposite Sides Theorem, the opposite sides of a parallelogram are congruent. Therefore, RS and QP are congruent. Additionally, by the Reflexive Property of Congruence, SP, or PS, is congruent to itself.
Consider the parallelogram PQRS and its diagonals PR and QS such that PR≅QS.
By the Parallelogram Opposite Sides Theorem, PQ≅SR. Additionally, by the Reflexive Property of Congruence, PS is congruent to itself.
This proof will use transformations to prove the theorem.
Consider the rectangle PQRS and its diagonals PR and QS. Let M be the point of intersection of the diagonals.
Let A and B be the midpoints of PS and RQ. Then, a line through M and the midpoints A and B can be drawn.
Reflection Across AB | |
---|---|
Preimage | Image |
R | Q |
S | P |
M | M |
Consider the parallelogram PQRS and its diagonals PR and QS such that PR≅QS. By the Parallelogram Diagonals Theorem, the diagonals of a rectangle bisect each other at M.
By the Parallelogram Opposite Sides Theorem, PQ≅SR and QR≅PS.
Let A and B be the midpoints of PS and RQ. Then, a line through M and the midpoints A and B can be drawn.
Zosia arrives early to a Harry Styles concert! She notices something about the stage, so she uses a napkin as paper and draws a diagram. The stage is a rectangle that she labels as ABCD.
In a rectangle, the diagonals are congruent.
As with rectangles, it can also be determined whether a parallelogram is a rhombus just by looking at its diagonals.
A parallelogram is a rhombus if and only if its diagonals are perpendicular.
Based on the diagram, the following relation holds true.
Parallelogram ABCD is a rhombus ⇔ <