Illustrative Mathematics
Equal Area Triangles on the Same Base II
A deceptively simple question setup leads to a number of attack methods and a surprisingly sophisticated solution set in this open-ended problem. Young geometers of different strengths can go about defining the solutions graphically,...
EngageNY
Angle Sum of a Triangle
Prove the Angle Sum Theorem of a triangle using parallel line and transversal angle relationships. Pupils create a triangle from parallel lines and transversals. They find angle measures to show that the angles of a triangle must total...
EngageNY
Construct an Equilateral Triangle (part 2)
Triangles, triangles, and more triangles! In this second installment of a 36-part series, your young mathematicians explore two increasingly challenging constructions, requiring them to develop a way to construct three triangles that...
Curated OER
The Truth About Triangles And Proofs
Students engage in a lesson plan that is about the classification of triangles and the mathematical proofs involved in working with them. They work on a variety of problems that are created by the teacher with the focus upon the...
EngageNY
Pythagorean Theorem, Revisited
Transform your pupils into mathematicians as they learn to prove the popular Pythagorean Theorem. The 16th lesson in the series of 25 continues by teaching learners how to develop a proof. It shows how to prove the Pythagorean Theorem...
EngageNY
Prove the Pythagorean Theorem Using Similarity
Amaze your classes with the ability to find side lengths of triangles immediately — they'll all want to know your trick! Learners use the Pythagorean Theorem and special right triangle relationships to find missing side lengths.
Geometry Accelerated
Accelerated Geometry Review Sheet
Your geometry learners use their knowledge of various geometric concepts to write proofs. Starting with givens containing parallel line segments with transversals and triangles and quadrilaterals, and the mid-point and distance formulas;...
EngageNY
Comparing the Ratio Method with the Parallel Method
Can you prove it? Lead your class through the development of the Side Splitter Theorem through proofs. Individuals connect the ratio and parallel method of dilation through an exploration of two proofs. After completing the proofs,...
Charleston School District
Pythagorean Theorem and Converse
You've heard that it is true, but can you prove it? Scholars learn the Pythagorean Theorem through proof. After an overview of proofs of the theorem, learners apply it to prove triangles are right and to problem solve. This is the second...
EngageNY
Law of Sines
Prove the Law of Sines two ways. The ninth segment in a series of 16 introduces the Law of Sines to help the class find lengths of sides in oblique triangles. Pupils develop a proof of the Law of Sines by drawing an altitude and a second...
Illustrative Mathematics
Placing a Fire Hydrant
Triangle centers and the segments that create them easily become an exercise in memorization, without the help of engaging applications like this lesson. Here the class investigates the measure of center that is equidistant to the three...
EngageNY
Properties of Parallelograms
Everyone knows that opposite sides of a parallelogram are congruent, but can you prove it? Challenge pupils to use triangle congruence to prove properties of quadrilaterals. Learners complete formal two-column proofs before moving on to...
Curated OER
A Different Proof of the Law of Cosines
In this proofs instructional activity, students construct 1 proof of the law of cosines. Students construct a right triangle within a circle to prove the law of cosines.
Curated OER
Right Triangle congruence
Students will determine if two triangles are congruent and which property is used. In this lesson on right triangles, students will determine what additional information is need to make a pair of triangles congruent.
Curated OER
Inequalities and Triangles
Students prove and apply theorems of triangle inequality and hinge theorem. In this geometry lesson, students will discuss which triangle is bigger and why. They will use theorems, such as Exterior Angle Inequality or the Pythagorean...
Curated OER
Right Triangles
Learners classify triangles and make connections regarding angle measurements and attributes of acute, right, and obtuse triangles. They listen to teacher define and explain triangles. Everyone imagines a place where a broken wing for a...
Curated OER
Geometry: Properties of Triangles
In this properties of triangles worksheet, students use their knowledge of triangles to find determine angle measurement. They use geometric theorems to prove equal angles. This three-page worksheet contains 15 multi-step geometry...
Curated OER
Pythagoras' Theorem
Students are introduced to the Pythagoras' Theorem and its history, proofs and practice in application. Students find perimeters, areas and volume of everyday objects. Students state and explain the theory.
Curated OER
Reflections and Equilateral Triangles
Your learners collaboratively find the lines of symmetry in an equilateral triangle using rigid transformations and symmetry. Through congruence proofs they show that they understand congruence in terms of rigid motions as they prove...
Curated OER
Proofs of the Pythagorean Theorem
Working individually and collaboratively, geometers gain a clear understanding of the Pythagorean theorem. They create, explain, and compare proofs of the theorem. Proofs involve areas of trapezoids, squares, and triangles, congruent...
GeoGebra
Conjectures and Proof
Transform proofs using triangle rotations. By rotating a triangle around various points, class members develop proofs. Participants prove relationships of alternative interior angles formed by parallel lines and the sum of the interior...
Chapman University
A Pythagorean-Style Proof of the Sine Sum-of-Angles Formula
This well organized poster shows a step-by-step algebraic proof and related graphic. Understanding how formulas were derived, connects learners to the idea that the study of mathematics is continuous and cumulative. Learners will also...
Curated OER
Special Segments in a Triangle
In this geometry worksheet, 10th graders review the vocabulary associated with the special segments of a triangle and the associated points of concurrency and solve problems in which they find the indicated missing angle or segment. The...
Curated OER
Self-Check for Triangle Congruence Proofs Multiple Choice
In this triangle congruence instructional activity, students select the postulate that is being illustrated to prove congruence between triangles. The instructional activity is self-scoring. Once students select the "check your work"...
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