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8.3 Tests for Parallelograms. Objectives. Recognize the conditions that ensure a quadrilateral is a parallelogram. Prove that a set of points forms a parallelogram in the coordinate plane. Conditions for a Parallelogram.

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## 8.3 Tests for Parallelograms

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**Objectives**• Recognize the conditions that ensure a quadrilateral is a parallelogram. • Prove that a set of points forms a parallelogram in the coordinate plane.**Conditions for a Parallelogram**• Obviously, if the opposite sides of a quadrilateral are parallel, then it is a parallelogram; but there are other tests we can also apply to a quadrilateral to test whether it is a parallelogram or not.**Conditions for a Theorems**• Theorem 8.9 – If both pairs of opposite sides are ≅, then the quad. is a . • Theorem 8.10 – If both pairs of opposite s are ≅, then the quad. is a . • Theorem 8.11 – If diagonals bisect each other, then the quad. is . • Theorem 8.12 – If one pair of opposite sides is ║ and ≅, then the quad. is a .**Given:**Proof: CPCTC. By Theorem 8.9, if both pairs of opposite sides of a quadrilateral are congruent, the quadrilateral is a parallelogram. Therefore, ABCD is a parallelogram. Example 1: Write a paragraph proof of the statement: If a diagonal of a quadrilateral divides the quadrilateral into two congruent triangles, then the quadrilateral is a parallelogram. Prove:ABCD is a parallelogram.**Given:**Your Turn: Write a paragraph proof of the statement: If two diagonals of a quadrilateral divide the quadrilateral into four triangles where opposite triangles are congruent, then the quadrilateral is a parallelogram. Prove:WXYZ is a parallelogram.**Proof:**by CPCTC. By Theorem 8.9, if both pairs of opposite sides of a quadrilateral are congruent, the quadrilateral is a parallelogram. Therefore, WXYZ is a parallelogram. Your Turn:**Example 2:**Some of the shapes in this Bavarian crest appear to be parallelograms. Describe the information needed to determine whether the shapes are parallelograms. Answer: If both pairs of opposite sides are the same length or if one pair of opposite sides is a congruent and parallel, the quadrilateral is a parallelogram. If both pairs of opposite angles are congruent or if the diagonals bisect each other, the quadrilateral is a parallelogram.**Your Turn:**The shapes in the vest pictured here appear to be parallelograms. Describe the information needed to determine whether the shapes are parallelograms. Answer: If both pairs of opposite sides are the same length or if one pair of opposite sides is congruent and parallel, the quadrilateral is a parallelogram. If both pairs of opposite angles are congruent or if the diagonals bisect each other, the quadrilateral is a parallelogram.**Example 3:**Determine whether the quadrilateral is a parallelogram. Justify your answer. Answer: Each pair of opposite sides have the same measure. Therefore, they are congruent. If both pairs of opposite sides of a quadrilateral are congruent, the quadrilateral is a parallelogram.**Your Turn:**Determine whether the quadrilateral is a parallelogram. Justify your answer. Answer: One pair of opposite sides is parallel and has the same measure, which means these sides are congruent. If one pair of opposite sides of a quadrilateral is both parallel and congruent, then the quadrilateral is a parallelogram.**Tests for Parallelograms**• Both pairs of opposite sides are parallel. • Both pairs of opposite sides are congruent. • Both pairs of opposite angles are congruent. • The diagonals bisect each other. • A pair of opposite sides is both parallel and congruent.**A**B D C Example 4a: Find x so that the quadrilateral is a parallelogram. Opposite sides of a parallelogram are congruent.**Example 4a:**Substitution Distributive Property Subtract 3x from each side. Add 1 to each side. Answer: When x is 7, ABCD is a parallelogram.**D**E G F Example 4b: Find y so that the quadrilateral is a parallelogram. Opposite angles of a parallelogram are congruent.**Example 4b:**Substitution Subtract 6y from each side. Subtract 28 from each side. Divide each side by –1. Answer: DEFG is a parallelogram when y is 14.**a.**b. Answer: Answer: Your Turn: Find m and n so that each quadrilateral is a parallelogram.**Parallelograms on the Coordinate Plane**• We can use the Distance Formula and the Slope Formula to determine if a quadrilateral is a parallelogram on the coordinate plane. • Just pick one of the tests… and apply either or both of the formulas.**Example 5a:**COORDINATE GEOMETRYDetermine whether the figure with vertices A(–3, 0), B(–1, 3), C(3, 2), and D(1, –1) is a parallelogram. Use the Slope Formula.**Answer: Since opposite sides have the same slope,**Therefore, ABCD is a parallelogram by definition. Example 5a: If the opposite sides of a quadrilateral are parallel, then it is a parallelogram.**Example 5b:**COORDINATE GEOMETRYDetermine whether the figure with vertices P(–3, –1), Q(–1, 3), R(3, 1), and S(1, –3)is a parallelogram. Use the Distance and Slope Formulas.**Example 5b:**First use the Distance Formula to determine whether the opposite sides are congruent.**Next, use the Slope Formula to determine whether**and have the same slope, so they are parallel. Example 5b: Answer: Since one pair of opposite sides is congruent and parallel, PQRS is a parallelogram.**Your Turn:**Determine whether the figure with the given vertices is a parallelogram. Use the method indicated. a. A(–1, –2), B(–3, 1), C(1, 2), D(3, –1); Slope Formula**Answer: The slopes of**and the slopes of Therefore, Since opposite sides are parallel, ABCD is a parallelogram. Your Turn:**Distance and Slope Formulas**b. L(–6, –1), M(–1, 2), N(4, 1), O(–1, –2); Your Turn: Determine whether the figure with the given vertices is a parallelogram. Use the method indicated.**Answer:**Since the slopes of Since one pair of opposite sides is congruent and parallel, LMNO is a parallelogram. Your Turn:**Assignment**• Pre-AP Geometry: Pg. 421 #13 – 32 • Geometry: Pg. 421 #4 – 7, 13 – 24, 25 – 28

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