What Topic is this?

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Across
  1. 3. A greek who liked to work with triangles
  2. 5. What is the longest side of a triangle called?
  3. 7. What is the best lesson in school?
  4. 9. What is the trigonometric ratio called that is represented by a/h
  5. 10. What is the study of triangles?
  6. 12. What is the Angle when looking upwards?
Down
  1. 1. What is the Angle when looking downwards?
  2. 2. What trigonometric ratio involves the O and A?
  3. 4. What is measured in degrees?
  4. 6. What is the trigonometric ratio called that begins with S?
  5. 8. What is the side of the triangle called that touches the angle?
  6. 11. What is the side of the triangle is opposite the angle?