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Olympiad workbook
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Olympiad workbook
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IOQM
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Sequences and Series

Olympiad workbook · IOQM · Sequences and Series

Let a_1=24 and form the sequence a_n, n ≥ 2 by a_n=100 a_ n-1 +134...

Let a1=24a_1=24 and form the sequence an,n≥2a_n, n \geq 2 by an=100an−1+134a_n=100 a_{n-1}+134. The first few terms are 24,2534,253534,25353534,………24,2534,253534,25353534, \ldots \ldots \ldots What is the least value of n for which an\mathrm{a}_{\mathrm{n}} is divisible by 99 ?

Correct answer

88

Step-by-step solution

a1=24&an=100.an−1+134a_1=24 \& a_n=100 . a_{n-1}+134 First few terms : a1=24a2=2534\begin{aligned} & a_1=24 \\ & a_2=2534 \end{aligned} a3=253534a4=25353534∴an=2535353……..534⏟(n−1) Times ′53′\begin{aligned} & a_3=253534 \\ & a_4=25353534 \\ & \therefore a_n=2 \underbrace{535353 \ldots \ldots . .534}_{(n-1) \text { Times }^{\prime} 53^{\prime}}\end{aligned} Now, an →\rightarrow divisible by 99⇒99 \Rightarrow by 9&119 \& 11 both Sum of digits =6+8(n−1)=6+8(n-1) To be divisible by 9 , n=7,16,25,34,43,52,61,70,79,88,….n=7,16,25,34,43,52,61,70,79,88, \ldots . a7=2535353535353⏟6 Times ′53′4\mathrm{a}_7=2 \underbrace{535353535353}_{6 \text { Times }{ }^{\prime} 53^{\prime}} 4 But a7→\mathrm{a}_7 \rightarrow Not divisible by 11 . a16=25353535353…..53⏟15 Times’53’ 4\mathrm{a}_{16}=2 \underbrace{5353535353 \ldots . .53}_{15 \text { Times'53' }} 4 Similarly, a16→\mathrm{a}_{16} \rightarrow Not divisible by 11 . Now, n=88\mathrm{n}=88 a88=25353……53⏟87 Times ′53′4\mathrm{a}_{88}=2 \underbrace{5353 \ldots \ldots 53}_{87 \text { Times }{ }^{\prime} 53^{\prime}} 4 \begin{aligned} & \text { Divisibility by } \begin{aligned} 11 & \rightarrow|(2+3+3 \ldots . .)-(5+5+\ldots .4)| \\ & =|263-439| \\ & =176\end{aligned} \\ & \therefore \text { Least } n=88\end{aligned}$
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