Let a_1=24 and form the sequence a_n, n ≥ 2 by a_n=100 a_ n-1 +134...
Let a1=24 and form the sequence an,n≥2 by an=100an−1+134. The first few terms are
24,2534,253534,25353534,………
What is the least value of n for which an is divisible by 99 ?
Correct answer
88
Step-by-step solution
a1=24&an=100.an−1+134
First few terms :
a1=24a2=2534a3=253534a4=25353534∴an=2(n−1) Times ′53′535353……..534
Now, an → divisible by 99⇒ by 9&11 both
Sum of digits =6+8(n−1)
To be divisible by 9 ,
n=7,16,25,34,43,52,61,70,79,88,….a7=26 Times ′53′5353535353534
But a7→ Not divisible by 11 .
a16=215 Times’53’ 5353535353…..534
Similarly, a16→ Not divisible by 11 .
Now, n=88a88=287 Times ′53′5353……534\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}$