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...
Khan Academy
Khan Academy: Triangle Angles Review
Reviews the rule that the interior angles of a triangle always add up to 180 degrees. Includes three practice problems.
CK-12 Foundation
Ck 12: Elem Math: Determining the Unknown Angle in a Triangle
[Free Registration/Login may be required to access all resource tools.] Students watch a video and attempt practice problems on how to determine a missing angle in a triangle.
CK-12 Foundation
Ck 12: Geometry: Triangle Sum Theorem Grade 7
[Free Registration/Login may be required to access all resource tools.] Recognize the sum of the interior angles of a triangle as 180 degrees.
CK-12 Foundation
Ck 12: Geometry: Triangle Sum Theorem Grades 8 10
[Free Registration/Login may be required to access all resource tools.] Recognize the sum of the interior angles of a triangle as 180 degrees.
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.
Mangahigh
Mangahigh: Shape: Proof of the Sum of the Angles in a Triangle
Determine that all angles in a triangle equal 180 degrees.
Walter Fendt
Walter Fendt: Sum of Angles in a Triangle
A short interactive activity to show that the sum of the angles of a triangle is 180 degrees.