Maths puzzle

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Across
  1. 4. The longest side of a right triangle. (10 letters)
  2. 6. The ratio of circumference to diameter of a circle.
  3. 11. A chord passing through the center of the circle. (9 letters)
  4. 13. The amount of 3D space a shape occupies.
  5. 14. A parallelogram with all sides equal. (7 letters)
  6. 17. The probability of an impossible event. (4 letters)
  7. 18. The point where two sides of a polygon meet. (6 letters)
  8. 19. A type of number system using only two digits: 0 and 1.
  9. 20. The branch of mathematics dealing with integers.
Down
  1. 1. The process of finding the derivative of a function. (11 letters)
  2. 2. The angle less than 90 degrees. (5 letters)
  3. 3. A 3D shape with two parallel circular bases.
  4. 5. The shape with a formula for surface area.
  5. 7. A line segment joining two points on a circle.
  6. 8. A straight line from the center to the circumference of a circle.
  7. 9. A flat surface extending infinitely in all directions. (5 letters)
  8. 10. area. The sum of the areas of all faces of a 3D object.
  9. 12. A part of a circle's circumference.
  10. 15. The value that occurs most frequently in a data set. (4 letters)
  11. 16. A system of numerals using ten digits.