Mathword puzzle

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Across
  1. 2. the distance around the circle
  2. 5. a problem with a product
  3. 7. a letter in an equation
  4. 9. a problem with a quotient
Down
  1. 1. a mathematical explanation involving equations and expressions
  2. 3. a 90 degree angle
  3. 4. a power that a number is raised by
  4. 6. a shape with at least 3 sides
  5. 8. half the distance to the middle of a circle