Interesting facts about integers

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Across
  1. 6. 1st culture to use negative numbers
  2. 7. (a*b) + c = (a*b) + (a*c) Guess the property
  3. 8. Multiplication of odd number of negative numbers
Down
  1. 1. a + b = c ; b + a = c Guess the property
  2. 2. Century when negative numbers were excepted into the number line
  3. 3. a + (b + c) = (a + b) + c = (a + c) + b Guess the property
  4. 4. abbreviation of Z which is the symbol of integers
  5. 5. multiplication of 2 negative integers
  6. 9. Sum of 2 odd numbers