The volume of a pizza of thickness 'a' and radius 'z' is given by pi*z*z*a.


        1*8 + 1 = 9
       12*8 + 2 = 98
      123*8 + 3 = 987
     1234*8 + 4 = 9876
    12345*8 + 5 = 98765
   123456*8 + 6 = 987654
  1234567*8 + 7 = 9876543
 12345678*8 + 8 = 98765432
123456789*8 + 9 = 987654321