Triangles and Quadrilaterals

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Across
  1. 2. Intersection of two of definition 7
  2. 4. Has a center of symmetry
  3. 6. Two opposite parallel sides
  4. 7. Their sum is always 180°
  5. 9. A circle whose center is the intersection of two of definition 12
  6. 12. The symmetry axis of an angle
  7. 13. A triangle with only one symmetry axis
  8. 16. Connects the midpoint of a side and the opposite vertex
  9. 19. Two diagonals lie outside!
  10. 20. The most perfect quadrilateral
  11. 21. 4 equal sides
Down
  1. 1. A concave kite
  2. 3. Pythagoras' favourite triangle!
  3. 5. 4 right angles
  4. 7. Perpendicular to one side and through the opposite vertex
  5. 8. All its angles measure 60°
  6. 10. A circle whose center is the intersection of two of the definition 11
  7. 11. The symmetry axis of a side
  8. 14. Three in a triangle
  9. 15. One diagonal is a symmetry axis
  10. 17. One diagonal lies outside
  11. 18. Nothing special!