Lesson plan

Use triangle congruence to prove and apply that rectangles are parallelograms with congruent diagonals

teaches Common Core State Standards CCSS.Math.Content.HSG-CO.C.11 http://corestandards.org/Math/Content/HSG/CO/C/11

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Big Ideas: Congruent triangles can be used to prove properties of parallelograms. Determine if the known properties of a quadrilateral can be used to prove a quadrilateral is a parallelogram. In this lesson students will observe that the diagonals of a rectangle appear to be congruent, and then use congruent triangles to prove this relationship. Vocabulary: parallelogram, rectangle, congruent, congruent triangles, diagonal, alternate exterior angles, opposite sides, opposite angles Special Materials: Cut outs of rectangles Ruler