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!