Maths puzzle

123456789
Across
  1. 3. last invented number
  2. 5. Magic number
  3. 7. negative numbers originated in
  4. 9. degree of circle
Down
  1. 1. sum of angles in triangle
  2. 2. base of decimal system
  3. 4. reads same forward and backward
  4. 6. multiplicative identity
  5. 8. middle number is