Math crossword

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Across
  1. 3. Which property is represented by a+b=b+a
  2. 6. Formulas that have the same logical values of the statements variables
  3. 7. The set of all real numbers that lie between two numbers a and b, a<b
  4. 8. "if p then q" (p=>q)
Down
  1. 1. A system in which the main role is played by the number two
  2. 2. Name of formula statements that are always true
  3. 4. "p or q" (p v q)
  4. 5. A narrative sentence that makes sense and is either true or false