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Alevel Math help! Sigma Notation. (Foundation programme).

Anonymous #1

How to find sigma formula when given a sequence?

Reply 1

username47781

I have moved this into the Maths section, you are more likely to get an answer here.

Reply 2

Htn_02

If you’re talking about sigma as in sum of a series up till a certain point then:
Determine what type of sequence it is - if arithmetic then its a + (n-1)d. If geometric then a X r^(n-1).
Then sub ur a and d values in and make them equal to the last value to find what the n value is ygm?
The number which goes on top of the sigma symbol is the last “n” value. The bottom is the first “n” value. Then beside that is your equation.
Hope this helped :smile:

Reply 3

Alisha999

Thank you! It makes sense but what do you mean by arithmetic and geometric...

My sequence is
11,13,15,17...31,33

The way I worked it out I got:
12 for the value on top of sigma sign and 1 for the bottom value and then in brackets the formula I got was (2n+9) :smile:

Not sure if I've worked it out correctly tho, I'm terrible at maths!

Thank you for your help :smile:

Reply 4

Htn_02

Original post by Alisha999

Not sure if I've worked it out correctly tho, I'm terrible at maths!

Thank you for your help :smile:

This one is arithmetic sequence because u add 2 each time. +/- means arithmetic, if 2,4,8,16 etc then geometric because you’re multiplying by a factor of 2.
And yup value on top is 12 and bottom 1.