Mathematics crossword

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Across
  1. 2. shape with four even sides
  2. 4. something minus something
  3. 6. shape with no sides
  4. 8. shape with three sides
Down
  1. 1. pay with it
  2. 3. something plus something
  3. 5. example: 1.8
  4. 7. example: 3/4