The Student Room Group
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>

C1 Sigma Notation help

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 ??:confused::confused::confused:
Reply 1
Avatar for mac_manutd
mac_manutd
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.
Reply 2
Avatar for Thrug
Thrug
3
Original post by applestrudle


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).



Yes, expand out r(r+1) and this makes r^2 + r, so the difference between them is getting bigger by 2 each time as it is r^2.
Reply 3
Avatar for Thrug
Thrug
3
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

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you