“Characteristics of Triangles”
Mr. Herald
Lesson Objectives:
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Prove or disprove “angle sum theorem”.
Prove or disprove the side-angle theorem of triangles.
Use technology to further our understanding of triangles.
Today we will learn about some characteristics and use Geometer’s SketchPad to prove or disprove
them.
1. Open a text box and type your name on the sketch.
2. Construct a scalene triangle.
3. Measure the length of each side and each angle. Drag the corresponding measurement next
to the side or angle.
4. Use SketchPad’s calculator to find the sum of the angles.
5. What is the sum of the angles? Does this prove or disprove our characteristics of all triangle
interior angles?
6. Type a statement which proves or disproves the sum of triangle angles.
7. Create another sketch and repeat steps 1 – 6 for an equilateral and create a third sketch for
an isosceles triangle.
8. Staple the three sketches together and turn them in before you leave today.
Questions to ponder:
Do you think we can make a triangle which doesn’t prove our triangle
angle statement?
Because all of our triangles prove this angle sum pattern, do you think we
could predict it for all triangles?
Did the number of congruent sides equal the number of congruent
angles in each triangle?
Because all of our triangles proved this side-angle relationship, do you
think we could predict it for all triangles?