# C1 sequences and seriesWatch

#1
This is a very stupid question, but when you use the sigma notation for working out the sum of something. eg. r=20 on top and r=5 on bottom and then something like (2r+4), what do you use for n? When r=1 on the bottom, n is the r on top, 20...but when r is 5??
0
12 years ago
#2
Its not a stupid question.

you would find the answer to your question ::r=20 on top and r=5 on bottom and (2r+4).

by working out

:r=20 on top and r=1 on bottom with (2r+4) and r=4 on top and r=1 on bottom with (2r+4); then subtract the Sums...

mrm.
0
#3
oh...i didnt know about that...why r=4 on top?
0
12 years ago
#4
As far as I'm aware, the C1 sum formulae only works from 1 to n, so if you had to go from five, I'd fudge it to something like this:

That'll get rid of the first four terms, leaving you with 5 <= n <= 20.
0
#5
For this particular example, they give r=30 on top and r=10 on bottom, with (7+2r). So a=27, d=2 and n=30-9=21...where did they get the 21 from? Am i going insane and not realising something really obvious?
0
12 years ago
#6
(Original post by kazaa)
oh...i didnt know about that...why r=4 on top?

so that you are left with the sum of terms 5 upto 20
0
12 years ago
#7
(Original post by kazaa)
For this particular example, they give r=30 on top and r=10 on bottom, with (7+2r). So a=27, d=2 and n=30-9=21...where did they get the 21 from? Am i going insane and not realising something really obvious?

ok there are 2 ways of doing these things either the sums as suggested above or indeed just using the sum of an arithmetic series.

From r=10 to r=30 means from (and including) the 10th to the 30th term. That makes 21 terms in total in general fro r=a to r=b will have b-a+1 terms in total
0
#8
Oooh I see! I completely forgot about that! Thanks!!
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