HELP PLEASE! Sum of a declining series

[shepdog]

Hello, hope someone can help with this...I'm trying enter a single cell fomula that will calculate the sum of a series that declines by a certain amount. Example:

T= 100, decline each time by i=1

so find: 100+(100-1)+(100-2) +(100-3)....+(100-100)


I know how to write it mathmatically, just can't figure out the excel equivalent to :

= T[1+ 'sum of i=1 to 100'(1+i)]


Thanks!

[MrShorty]

That kind of open ended sum will be challenging to formulate in a single cell. Easiest will be to make use of the formula sum(i=1 to n)(i)=(n^2+n)/2. Proof of this formula is usually found in some high school math texts (can't remember if it was geometry or trig or what specific course), often associated with a discussion of triangle numbers or proof by induction.