Arithmetic means

Algebra Level 3

2 , , 6053 2, \ldots, 6053

Consider an arithmetic progression of ( n + 2 ) (n+2) terms with first term 2 and last term 6053.

If the ratio between the second term and the ( n 4 ) th (n-4)^\text{th} term is 1 : 1207 1:1207 , find the value of n n .


The answer is 2016.

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1 solution

Ashish Menon
May 25, 2016

Let d d be the common difference, a a be the first term and l l be the last term.
d = l a n + 1 = 6053 2 n + 1 = 6051 n + 1 d = \dfrac{l-a}{n+1} = \dfrac{6053-2}{n+1} = \dfrac{6051}{n+1}
A 1 A_1 (first arithmetic mean) = a + d = 2 + 6051 n + 1 = 2 n + 6053 n + 1 a + d = 2 + \dfrac{6051}{n+1} = \dfrac{2n + 6053}{n+1} .
A n 5 A_{n-5} (first arithmetic mean) = a + ( n 5 ) d = 2 + 6051 ( n 5 ) n + 1 = 6053 n 30253 n + 1 a + (n-5)d = 2 + \dfrac{6051(n-5)}{n+1} = \dfrac{6053n - 30253}{n+1} .


A 1 A n 5 = 2 n + 6053 n + 1 × n + 1 6053 n 30253 = 1 1207 2 n + 6053 6053 n 30253 = 1 1207 1207 ( 2 n + 6053 ) = 6053 n 30253 2414 n + 7305971 = 6053 n 30253 3639 n = 7336224 n = 7336224 3639 n = 2016 \dfrac{A_1}{A_{n-5}} = \dfrac{2n + 6053}{n+1} × \dfrac{n+1}{6053n - 30253} = \dfrac{1}{1207}\\ \dfrac{2n + 6053}{6053n - 30253} = \dfrac{1}{1207}\\ 1207\left(2n + 6053\right) = 6053n - 30253\\ 2414n + 7305971 = 6053n - 30253\\ 3639n = 7336224\\ n = \dfrac{7336224}{3639}\\ n = \color{#69047E}{\boxed{2016}} .

It is A 1 A_1 and not A 3 A_3 in your 4th step.

Ayush G Rai - 5 years ago

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Thanks I edited it.

Ashish Menon - 5 years ago

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I got the answer as 4.

Ayush G Rai - 5 years ago

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@Ayush G Rai Answer can never be 4, then the (n-5)th arithmetic mean never makes sense. If there is only 4 arithmetic means what is (4-5=-1)th arithmetic mean?

Ashish Menon - 5 years ago

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@Ashish Menon sorry again!!

Ayush G Rai - 5 years ago

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@Ayush G Rai :) Mistakes are done only by humans :) You would get it correct next time.

Ashish Menon - 5 years ago

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@Ashish Menon yeah!!!!!!!!!!!

Ayush G Rai - 5 years ago

Nice Question..+1

Sabhrant Sachan - 5 years ago

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You have understood the word "arithmetic means~ right?

Ashish Menon - 5 years ago

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yeah , i understand English -_-

Sabhrant Sachan - 5 years ago

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@Sabhrant Sachan No offence bro, but view reports. He tells there is only one arithmetic mean.

Ashish Menon - 5 years ago

but instead of writing : "In between 2 and 6053 is inserted n n arithmetic means" , write : " between 2 and 6053 n n arithmetic means are inserted"

Sabhrant Sachan - 5 years ago

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@Sabhrant Sachan Pi Han told that he would do that for me. ¨ \ddot\smile

Ashish Menon - 5 years ago

0 pending reports

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