stuck maths Watch

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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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(Original post by three123)
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Leave this with me.
I'll PM the answer
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(Original post by Nunchuck-master-2334)
Leave this with me.
I'll PM the answer
thanks but could explain it because i need an A in maths :'( very unlikely
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(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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(Original post by Nunchuck-master-2334)
Don't say that!
I got a high B and didn't revise!
can you pm me please
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(Original post by three123)
can you pm me please
Let me work the answer out first.
Will be a while.
Am in A-Level Computing at the mo'
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RDKGames
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(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 u_r=7+2r so first term is 9 and common difference is 2, use the formula for finite summation of a arithmetic sequence.

To start you off, \displaystyle \sum_{r=10}^{30} (7+2r) = \sum_{r=1}^{30} (7+2r) - \sum_{r=1}^{9} (7+2r)
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(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 u_r=7+2r so first term is 9 and common difference is 2, use the formula for finite summation of a arithmetic sequence.

To start you off, \displaystyle \sum_{r=10}^{30} (7+2r) = \sum_{r=1}^{30} (7+2r) - \sum_{r=1}^{9} (7+2r)
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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(Original post by Nunchuck-master-2334)
Leave this with me.
I'll PM the answer
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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(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.

S_n=\frac{n}{2}(a+l) is the summation of an arithmetic sequence from the first term up to the n^{th} term, with a denoting the first term of the sequence, and l denoting the final term of the sequence.

In your example, you are being asked to find the sum from the 10^{th} term up to the 30^{th} term for the sequence u_r=7+2r. 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 \infty and even then, you cannot apply S_{\infty}=\frac{a}{1-r} because that one is for a geometric series and yours is arithmetic as I've mentioned...
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