Consider this triangle made out of two-sided blue and yellow origami paper. [list] [*]Drag the blue vertices to adjust the shape of the triangle. [/list] First we will construct the altitude through the upper vertex. [list] [*]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. [/list] The crease (the red doted line) is the altitude to the base. Why? [list] [*]Drag each vertex of the triangle to the red point (the foot of the altitude). [/list] What can you say about the sum of the interior angles of this triangle? [list][*]Click reset and experiment with different triangles.[list] [/list]