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# If m is the average (arithmetic mean) of the first 10

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Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 10220
Location: Pune, India
Re: If m is the average (arithmetic mean) of the first 10  [#permalink]

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24 Feb 2018, 04:29
1
dave13 wrote:

what is difference between this formula For an arithmetic prog, Average = (first term + last term)/2 and this one $$x_n = a + d(n−1)$$ ?

happy weekend

The two formulas are to find 2 different things.

Average = (first term + last term)/2 gives you the average of the AP. It could be the middle term if you have odd number of terms in the AP or it will be the average of two middle terms if you have even number of terms in the AP

$$x_n = a + d(n−1)$$ gives us the nth term.

e.g.
AP = 3, 6, 9, 12, 15
a = first term = 3
d = common diff = 3
n = number of terms = 5

Average = (15 + 3)/2 = 9 (the middle term)

Second term = 3 + 3*(2 - 1) = 6
Third term = 3 + 3*(3 - 1) = 9
Fourth term = 3 + 3*(4 - 1) = 12
and so on...
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Karishma
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Re: If m is the average (arithmetic mean) of the first 10  [#permalink]

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14 Mar 2019, 11:30
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Re: If m is the average (arithmetic mean) of the first 10   [#permalink] 14 Mar 2019, 11:30

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