Recursive sequence

Algebra Level 4

We define a recursive sequence of natural numbers such that a 1 = 2 a_{1}=2 and a n + 1 = 3 a n + 2 a_{n+1}=3a_{n}+2 for n 1 n\geq1 . Find [ 2 n = 1 100 a n ] + 200 a 100 \frac{[2\sum_{n=1}^{100}a_{n}]+200}{a_{100}} .

The problem is original.


The answer is 3.

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2 solutions

Surya Prakash
Jul 27, 2015

a n + 1 = 3 a n + 2 a_{n+1} = 3 a_{n} +2

i = 1 n a i + 1 = 3 i = 1 n a i + 2 n \sum_{i=1}^{n} a_{i+1} = 3 \sum_{i=1}^{n} a_{i} + 2n

a n + 1 + i = 1 n a i a 1 = 3 i = 1 n a i + 2 n a_{n+1} + \sum_{i=1}^{n} a_{i} - a_{1} =3 \sum_{i=1}^{n} a_{i} + 2n

Let S = i = 1 100 a i S= \sum_{i=1}^{100} a_{i} and substitute n = 100 n=100 .

So, a 101 + S a 1 = 3 S + 200 a_{101} + S -a_{1} = 3S + 200

3 a 100 + 2 2 = 2 S + 200 3a_{100} + 2 -2 = 2S +200

2 S + 200 = 3 a 100 2S + 200 = 3 a_{100}

Therefore, 2 i = 1 n a i + 200 a 100 = 3 \frac{2 \sum_{i=1}^{n} a_{i} +200}{a_{100}} = \boxed{3}

Moderator note:

Great usage of the recurrence relation.

I would expect that most people tried to solve the recurrence, and substituted those values instead.

good work, a small typo in the third last step it is 2 S + 200 2S+200

Tanishq Varshney - 5 years, 10 months ago

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yup!! corrected it.

Surya Prakash - 5 years, 10 months ago
Sean Sullivan
Jul 27, 2015

a n + 1 = 3 a n + 2 a_{n+1}=3a_{n}+2

a n + 1 + 1 = 3 a n + 3 = 3 ( a n + 1 ) a_{n+1}+1=3a_{n}+3=3(a_{n}+1)

Let b n = a n + 1 b_{n}=a_{n}+1 then b n + 1 = 3 b n b_{n+1}=3b_{n} and b 1 = 3 b_{1}=3 so b n = 3 n b_{n}=3^{n}

Now we see a n = 3 n 1 a_{n}=3^{n}-1

[ 2 n = 1 100 ( 3 n 1 ) ] + 200 3 100 1 = [ 2 3 ( 3 100 1 ) 2 200 ] + 200 3 100 1 = 3 \frac{[2\sum\limits_{n=1}^{100}(3^{n}-1)]+200}{3^{100}-1}=\frac{[2\frac{3(3^{100}-1)}{2}-200]+200}{3^{100}-1}=\boxed{3}

nice approach

Surya Prakash - 5 years, 10 months ago

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