sweetiemelx
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#1
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#1
How would you work out something like?

55
∑ (7 + 2r)
r=10
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Zacken
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#2
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#2
(Original post by sweetiemelx)
How would you work out something like?

55
∑ (7 + 2r)
r=10
Write it as \displaystyle \sum_{r=1}^{55} (7+2r) - \sum_{r=1}^{9} (7+2r) and evaluate both separately where both are standard arithmetic sequences.
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sweetiemelx
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#3
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#3
(Original post by Zacken)
Write it as \displaystyle \sum_{r=1}^{55} (7+2r) - \sum_{r=1}^{9} (7+2r) and evaluate both separately where both are standard arithmetic sequences.
Thank you and what if r=2 or 3
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Zacken
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#4
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#4
(Original post by sweetiemelx)
Thank you and what if r=2 or 3
Not sure what you mean, you're summing over the relevant r.
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LastMinReviseGuy
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#5
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#5
What the what!!!
What exam board is this?
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iMacJack
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#6
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#6
(Original post by LastMinReviseGuy)
What the what!!!
What exam board is this?
Edexcel
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LastMinReviseGuy
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#7
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#7
Where can i find questions like this?
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PossiblyNotGod
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#8
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#8
For this question I think it's quicker to let
a = 7 + 2(10),
L = 7+2(55)
and use Sn = (n/2)(a + L) = (46/2)(27 + 117) = 3312.
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LastMinReviseGuy
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#9
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#9
Ah i see, does not seem so hard after looking at for a min or 2.
Thanks XOR for writing down your calculations.
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LastMinReviseGuy
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#10
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#10
Do they give you both arithmetic sequences formulas?
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Zacken
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#11
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#11
(Original post by LastMinReviseGuy)
Do they give you both arithmetic sequences formulas?
Nopes. But it's not hard to remember n/2(first term + last term). Or you can derive one from the other.
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