Let the sum of the following infinite sequence be N . 1 , 2 1 , 2 1 , 6 1 , 4 1 , 1 2 1 , 8 1 , 2 0 1 , 1 6 1 , 3 0 1 , 3 2 1 , 4 2 1 … . Find the remainder when ( N 2 0 1 4 + 1 ) is divided by 11 .
Hint :- Is it really a sequence?
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well, we can also do 3 2 0 1 4 by fermat's little theorem! I did the rest exactly the same! Nice!
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Would you please explain how .I know the theorem but how to apply it here?
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3 1 0 = 1 mod 11
3 2 0 1 0 = 1 mod 11
Now, we have to find 3 4 or 81 mod 11 which is just 4.
They say solving math problems gives one a rush of adrenaline. This one is not for the faint of heart then!! Really nice problem and amazing solution.
nice problem @Aditya Raut
nice one bro
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When you like a problem/solution/note (not just mine, any other you come across...) , then if you admire it, hit the like, reshare , upvote buttons to show your appreciation. That's a better option for this ! Thank you.
@Aditya Raut...NICE EXPLAINATION!! But, in the telescoping series, how are you left with just the '1'...why not the last term as well? Is it because we don't know what is it?
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It is because the last term is 1/infinity=0
Infinite series, limits, divergence!!
Zayam Alam said it right, see that as the series goes further, the denominator goes on increasing, the overall number increasing. If you go on continuing, as he said, at very very large denominators, the terms will be tending to 0 and because it's till infinity, ∞ 1 = 0 , makes the sum 1 .
Aargh!! Stupid dumb me!! @Aditya Raut -I got all of this, wasted two of my tries in finding the remainder part and gave up! Should've computer-bashed it!! :(
Very nice problem. The series part has a couple of nice parts to go around :)
Nice problem! Arranged the 2 infinity sums a little different, nice with the geometric sequences though. Haven't really seen that before!
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See that the sequence is actually inter-twined mixing of 2 sequences 1 , 2 1 , 2 1 , 6 1 , 4 1 , 1 2 1 , 8 1 , 2 0 1 , 1 6 1 , 3 0 1 , 3 2 1 , 4 2 1 . . . .
Out of these, the sequence in Blue is S ∞ = 1 + 2 1 + 4 1 + 8 1 . . . which is a Geometric progression (GP) with a = 1 , r = 2 1 , so it's infinite sum is
∙ 1 − r a = 1 − 2 1 1 = 2 1 1 = 2
The sequence in Brown is 1 × 2 1 + 2 × 3 1 + 3 × 4 1 + 4 × 5 1 . . . . .
This can be written as
1 × 2 2 − 1 + 2 × 3 3 − 2 + 3 × 4 4 − 3 + 4 × 5 5 − 4 + . . . .
= ( 1 − 2 1 ) + ( 2 1 − 3 1 ) + ( 3 1 − 4 1 ) + ( 4 1 − 5 1 ) + . . . . A telescoping series = 1
This gives N = 3 .
We want to find 3 2 0 1 4 + 1 ( m o d 1 1 )
Observe that
3 5 = 2 4 3 ≡ 1 ( m o d 1 1 ) ⟹ 3 5 k ≡ 1 ( m o d 1 1 ) ∴ 3 2 0 1 0 ≡ 1 ( m o d 1 1 ) ∴ 3 2 0 1 4 ≡ 3 4 ≡ 8 1 ≡ 4 ( m o d 1 1 )
Thus, asked answer will be 3 2 0 1 4 + 1 ≡ 4 + 1 ≡ 5 ( m o d 1 1 )