Trig-Hard

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Across
  1. 5. Angle above the horizontal
  2. 6. Occurring in intervals
  3. 9. Maximum distance from a sinusoidal curve's midline
  4. 11. The time between oscillations
  5. 13. cos 2θ = cos^2 θ − sin^2 θ
  6. 15. Formula that relates any triangle's angles to its side lengths
  7. 16. Relating to circles
  8. 18. Of or relating to sines
  9. 19. Reciprocal of cosine
Down
  1. 1. Angle below the horizontal
  2. 2. To be [] to something means to be perpendicular to it
  3. 3. sin θ/2 = ±√[(1 - cos θ) / 2]
  4. 4. This formula uses a triangle's side lengths to find its area
  5. 7. An identity derived from the triangular definitions of sine and cosine
  6. 8. Reciprocal of period
  7. 10. The positive acute angle between an angle's terminal side and the x axis
  8. 12. - tan (θ - pi/2)
  9. 14. Pythagorean theorem modified for non-right triangles
  10. 17. Reciprocal of sine