okay basically this is the question:
its the sigma sight with 5 on the top, r= 0 on the bottom and the equation things is r(r+1).
I keep getting 70 but the answers at the back of the book say its 112.
zeroth term, 1st term, 2nd term, 3rd term, fourth term, fifth term.
0 + 2 + 6 + 12 + 20 + 30
I didn't use sn = 0.5n (2a +(n-1)d) becuase the difference keeps on changing (so there is no common difference ? not sure).
So yeahh, what am i doing wrong ??
C1 Sigma Notation help Watch
- Thread Starter
- 28-11-2010 02:35
- 28-11-2010 02:41
Well if the question is exactly as you say then yes 70 is the right answer, 112 comes from doing the sum from r=1 to 6 so either youve read the question wrong or the book is wrong.
- 28-11-2010 12:02
- 28-11-2010 18:27
From looking at my stats I just found the formula which can be used for when there is a ^2 question:
Sn = n/6 (n+1)(2n+1)
So with 5 terms, n=5 :
5/6 * 6 * 11 = 55
But it's r^2 +r, so add on 5+4+3+2+1 = 15
So 55 + 15 = 70