Einstein's Pie

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Across
  1. 2. why be together
  2. 4. times is the key
  3. 6. is measured in square units
  4. 7. historian of pi
  5. 8. is not spelled like "pie"
  6. 9. Nasa uses pi every day to calculate the size needed for the __________ to work correctly
Down
  1. 1. the entirety of the circle
  2. 2. Pi equals the circumference divided by the ________
  3. 3. is always March 14th
  4. 5. something that never ends