Calculus
posted by Luis .
Express the following in Sigma notation
2+8+18+32+50+72

Calculate the difference between successive terms of the sum. The differences are the terms you need to add together to make the sum.
Sum: 2,8,18,32,50,72...
diff: 2,6,10,14,18,22...
0th term=2
1st term = 6
2nd term = 10
The rule is therefore 2+4i
So check if Σ2+4i works for you. i starts from zero. 
My answer above applies if the list of numbers were successive sums.
Go with Reiny's answer:
http://www.jiskha.com/display.cgi?id=1262752887