Mathematical week

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Across
  1. 3. What does the term "perimeter" mean?
  2. 4. The opposite of addition?
  3. 8. Type of equations where unknowns are related to moments in time?
  4. 9. First even number?
Down
  1. 1. What does the number π mean?
  2. 2. Which number is the neutral element in addition?
  3. 4. What is the name for the process of finding the value of a variable in an equation?
  4. 5. Equation with a binary unknown?
  5. 6. How many angles does a hexagon have?
  6. 7. Geometric shape with one line of symmetry and two edges?