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each term is obtained by adding 4 to the preceding one

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Junior | Next Rank: 30 Posts
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by imago » Tue Sep 18, 2007 4:07 pm
In a sequence, each term is obtained by adding 4 to the preceding one. If the sum of the first 10 terms is equal to 80, what is the result of the addition of the first 40 terms?

94
320
2,720
27,200
54,400


OA:C
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by kajcha » Tue Sep 18, 2007 5:21 pm
Ans C

This is AP. Sum of n terms in AP is given by

S(n) = n/2(2a+(n-1)d)

First part of q says

80 = 10/2(2a+(10-1)4) => a = -10

Now S(40) = 20(2*-10+(40-1)4) = 2720
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by dpatwa » Tue Sep 18, 2007 10:37 pm
Where is S(n) = n/2(2a+(n-1)d) from? what property is it?
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by kajcha » Wed Sep 19, 2007 3:00 am
Check this site

https://www.mathsrevision.net/alevel/pages.php?page=33
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