Question 258380
day joined membership
0    0    0
1 1 1
2 2 3
3 3 6
4 4 10
5 5 15
6 6 21
7 7 28
8 8 36
9 9 45
10 10 55
11 11 66
12 12 78
13 13 91

The new total  is the previous total plus the new day
#8 is 36 which is 1/2*8*9
#10 is 55 which is 1/2*10*11=55
#12 78 1/2 12*13=6*13
total = day*day+1*1/2
It also works with odd numbers
#11 11*12*1/2=66
so we have 
t=d*(d+1)*1/2
t=40*41*1/2
t=20*41=820 after 40 days