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The Sum of the Interior Angles of Any Triangle
Author:
Irina Boyadzhiev
Topic:
Angles
Consider this triangle made out of two-sided blue and yellow origami paper.
Drag the blue vertices to adjust the shape of the triangle.
First we will construct the altitude through the upper vertex.
Click the “Fold” button. This will color the lower left vertex yellow.
Drag the the yellow vertex until the “crease” passes through the upper vertex.
Click on the yellow triangle to “crease”.
Unfold the triangle completely.
The crease (the red doted line) is the altitude to the base. Why?
Drag each vertex of the triangle to the red point (the foot of the altitude).
What can you say about the sum of the interior angles of this triangle?
Click reset and experiment with different triangles.[list]
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