Each equation below contains the initials of words that are needed to complete it. Find the missing words. All of the equations are mathematical (and some involve trigonometry).
For example, 36 = I. in a Y. would be 36 = Inches in a Yard.

  1. ______________         0   =   the T. of P. R.
  2. ______________         0   =   the I. E. for A.
  3. ______________           =   the C. of F.-F. D.
                         
  4. ______________         1   =   the S. of N. D.
  5. ______________         1   =   the I. E. for M.
  6. ______________           =   the H. of an I. R.T. whose L. E. O.
                         
  7. ______________         2   =   the C. of T. D.
  8. ______________         5   =   the D. in a P.
  9. ______________         9   =   the S. of a N.
                         
  10. ______________         10   =   M. in a C.
  11. ______________         12   =   V. in a D.
  12.   Inches in a Yard       36   =   I. in a Y.
                         
  13. ______________         45   =   A. of   I. of a L. with S. E. to O.
  14. ______________         49   =   the S. of S.
  15. ______________         60   =   D. in E. A. of an E. T.
                         
  16. ______________         90   =   S. of the A. A. of a R. T.
  17. ______________         90   =   D. in a R. A.
  18. ______________         100   =   D. C. at which W. B.
                         
  19. ______________         120   =   D. in E. I. A. of a R. H.
  20. ______________         360   =   D. in a C.
  21. ______________         640   =   A. in a S. M.