# stuck mathsWatch

#1
i thought i was supposed to use the sum of infinity but the mark scheme uses a and l (first and last terms) why is this confusing
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#2
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2 years ago
#3
(Original post by three123)
Leave this with me.
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#4
(Original post by Nunchuck-master-2334)
Leave this with me.
thanks but could explain it because i need an A in maths :'( very unlikely
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2 years ago
#5
(Original post by three123)
thanks but could explain it because i need an A in maths :'( very unlikely
Don't say that!
I got a high B and didn't revise!
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#6
(Original post by Nunchuck-master-2334)
Don't say that!
I got a high B and didn't revise!
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2 years ago
#7
(Original post by three123)
Let me work the answer out first.
Will be a while.
Am in A-Level Computing at the mo'
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#8
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2 years ago
#9
(Original post by three123)
i thought i was supposed to use the sum of infinity but the mark scheme uses a and l (first and last terms) why is this confusing
You can't really use the summation to infinity when you're not summing up infinite terms.

Your sequence is given by so first term is 9 and common difference is 2, use the formula for finite summation of a arithmetic sequence.

To start you off,
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#10
(Original post by RDKGames)
You can't really use the summation to infinity when you're not summing up infinite terms.

Your sequence is given by so first term is 9 and common difference is 2, use the formula for finite summation of a arithmetic sequence.

To start you off,
so would we use sum of infinity if there wasn't a number on top of sigma?

i'm using Sn= n/2 (a+l)
a= 27
l= 67

but what is n? im not sure what Sn is
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2 years ago
#11
(Original post by Nunchuck-master-2334)
Leave this with me.
Please don't give full solutions, whether it be by PM or as a thread reply. It's much more beneficial to guide the student towards the answer or request their working so you can see what they've done wrong / help them with the next step.

Plus it's against the rules to give full solutions.
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2 years ago
#12
(Original post by three123)
so would we use sum of infinity if there wasn't a number on top of sigma?

i'm using Sn= n/2 (a+l)
a= 27
l= 67

but what is n? im not sure what Sn is
Have you properly covered this topic? The questions you are asking are elementary.

is the summation of an arithmetic sequence from the first term up to the term, with denoting the first term of the sequence, and denoting the final term of the sequence.

In your example, you are being asked to find the sum from the term up to the term for the sequence . I have given you a head start on the problem by splitting the sum in such a way that allows you to apply this formula.

Regarding summation to infinity, you would only apply the formula when the upper limit is and even then, you cannot apply because that one is for a geometric series and yours is arithmetic as I've mentioned...
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