Curated OER
Triangle's Interior Angles
Given a pair of parallel lines and a triangle in between, geometers prove that the sum of the interior angles is 180 degrees. This quick quest can be used as a pop quiz or exit ticket for your geometry class.
Illustrative Mathematics
Sum of Angles in a Polygon
How can learners use algebra to solve a geometry problem? Help learners create an equation that shows the relationship between the number of sides of a polygon and the sum of the interior angles. Students are asked to divide the...
Illustrative Mathematics
Right Triangles Inscribed in Circles I
One of the basic properties of inscribed angles gets a triangle proof treatment in a short but detailed exercise. Leading directions take the learner through identifying characteristics of a circle and how they relate to angles and...
Curated OER
Why Does ASA Work?
Your geometry learners explore Angle-Side-Angle congruence in this collaborative task. The sum of the interior angles of all triangles being one hundred eighty degrees, is the key learners will discover as they explain their reasoning...
Texas Instruments
Texas Instruments: Angle Sum in Triangles Proof: Rotation and a Parallel Line
This investigation uses Cabri Jr. and a cleaver rotation of a triangle to "prove" that the angles in a triangle add up to 180. This could be used to reinforce triangles and paralled lines as well as introduce the concept of rotating an...
Texas Instruments
Texas Instruments: Extension: Parallel Lines and the Sum of the Angles
Students construct a line parallel to the base of a triangle and measure the angles formed. They use the construction to prove that the sum of the measures of the interior angles of a triangle is 180 degrees.
Math Is Fun
Math Is Fun: Triangles Contain 180 Degrees
Students learn about the angle-sum theorem. A definition, example, and proof are included.