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Question regarding arithmetic difference is a - b / 2? or only a - b?

I think the answer choice should be 40 * 3^11. Please do check the source and correct me if I am wrong! Also, the fact that we are calculating the difference of the 13th and the 11th term since this is an increasing GP!

The first term or the a0 = 5 and a1 = 15. For geometric progression, a1 = r * a0 where r is the common term. Hence, r comes out to be 3.

My apologies! Since I started of the series with a0 I completely screwed up the counting! As you have mentioned, the 13th term will have the 12th power and the 11th term will have the 10th power! *precisely the difference between choice [B] and [D]*

And as you have mentioned, the arithmetic difference is the the simple difference of the two terms leading to the answer [D]!

Question regarding arithmetic difference is a - b / 2? or only a - b?

For a given Geometric Sequence, the \(n^{th}\) term is \(t_n = a*r^{n-1}\), where a is the first term and r is the common ratio. From the given problem, \(t_{11} = 5*3^{10}\) and \(t_{13} = 5*3^{12}\). Thus, the arithmetic difference is : \(5*3^{10}*(9-1)\) = \(40*3^{10}\).
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Re: 5 and 15 are the first two terms in a geometric sequence. [#permalink]

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24 Jan 2016, 21:27

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Re: 5 and 15 are the first two terms in a geometric sequence. [#permalink]

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