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Across 4. An example used to prove that an if-then statement is false 6. A statement that contains the words "if and only if" 7. A triangle with all angles congruent 12. Two angles whose sides form two pairs of opposite rays 16. Circles that lie in the same plane and have the same center 18. A 10-sided polygon 19. A statement that is accepted without proof

Down 1. Two angles whose measures have the sum 180 2. The perpendicular segment from a vertex to the line containing the opposite side 3. The figure formed by three segments joining three noncollinear points 4. A segment whose endpoints lie on a circle 5. A statement that can be proved 8. Two angles whose measures have the sum 90 9. A line that contains a chord 10. Two angles in a plane that have a common vertex and a common side but no common interior points 11. A triangle with no sides congruent 13. A line that intersects two or more coplanar lines in different points 14. A chord that contains the center of a circle 15. A triangle with at least two sides congruent 17. A quadrilateral with four congruent sides