Algebra Puzzle

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Across
  1. 1. middle value in a set
  2. 9. the horizontal axis
  3. 11. not expressible as a fraction
  4. 13. most frequent value
  5. 14. input values of a function
  6. 15. expressible as a fraction
Down
  1. 2. not forming a straight line
  2. 3. a statement of equality
  3. 4. a relation with unique outputs
  4. 5. a symbol representing a value
  5. 6. forming a straight line
  6. 7. steepness of a line
  7. 8. average of a set of numbers
  8. 10. the vertical axis
  9. 12. output values of a function