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### Geometry Puzzles!

posted Feb 10, 2015, 9:33 AM by Michael Dreyfus-Pai   [ updated Sep 11, 2016, 3:03 PM by kmcmillan@ctkschool.org ]
Here are some geometry activities you can try out next:

At KhanAcademy.org you will find some recommended skills to practice.

### Proofs

Proofs are like puzzles. To prove something in geometry, you can only use other geometry facts you already know. You can't use measuring in a proof!

1) Angles in a trapezoid

• Draw a pair of parallel lines
• Draw TWO transversals. Do not connect them.
• Pairs of supplementary angles
• What's the sum of the interior angles? How can you know this WITHOUT MEASURING THEM?

2) Angles in a triangle

• Using lines (not line segments), draw a triangle
• Select a vertex and the line opposite that vertex. Construct a parallel line.
• Using what you know about parallel lines cut by a transversal, find the two ? angles.
• What is the sum of the interior angles of the triangle? How can you know this without measuring them?