Let α and β be the roots of x 2 − 6 x − 2 = 0 , with α > β . If a n = α n − β n for n ≥ 1 , then k = 2 a 9 a 1 0 − 2 a 8 Then find the remainder when k 9 4 2 is divided by 2 0 1 4 .
Note
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Adding 1 0 0 0 in the answer will give 1 7 2 9 called Ramanujan Number. It is the smallest natural number which can be written as the sum of cubes of two distinct un-ordered pairs. 1 7 2 9 = ⎩ ⎪ ⎨ ⎪ ⎧ 1 2 3 + 1 3 1 0 3 + 9 3
To find the remainder, we can apply Euler's theorem. Since 2 0 1 4 = 2 × 1 9 × 5 3 we have ϕ ( 2 0 1 4 ) = 9 3 6 . Hence 3 9 4 2 ≡ 3 6 = 7 2 9 ( m o d 2 0 1 4 ) .
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Yeah did the same!
Thanks @Jubayer Nirjhor. Actually I did not know the factors of 1007.
Can you please write a solution for calculating the remainder.
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I asked @Aneesh kundu the same and he replied the remainder would be 1 . No problem i was able to get but was unable to find remainder so its okay.
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Bro don't lose the spirit , i know your are a genius.
Try this one , you also @souvik paul
Find the remainder when 1 6 9 0 2 6 0 8 + 2 6 0 8 1 6 9 0 is divided by 7
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@U Z – @megh choksi is your question's answer 0????
Sorry but I actually mistook ϕ ( 2 0 1 4 ) to be 942 instead of 936. I've edited it now. Sorry if u lost any points because of me.
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@Aneesh Kundu – No problem because i was unable to get the remainder. Its okay .Dont be sorry.
Truly amazing! You're Awesome, I'm awestruck, I bashed it all.
Use Newton's sum for simplifying 2 a 9 a 1 0 − 2 a 8 into
2 a 9 a 9 ( a ) + 2 a 8 − 2 a 8
= 2 a 9 a ( a 9
= 3
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What is newton sums? @Kartik Sharma
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a n + b n = ( a + b ) ( a n − 1 + b n − 1 ) − a b ( a n − 2 + b n − 2 )
Crap, forgot the coefficient of 2 in the denominator and got 334. D:
Just saying, the 'note' is a pure spoiler to the question. One may simply guess the answer. I suggest you to remove it.
I solved it using newton sum and using identities. Your solution is awesome.
Awesome!!! Best solution.
Since α and β are roots of the equation: α β = − 2 α + β = 6
Now, multiplying and dividing by 3 and substituting the above values in the given expression, we get: k = 2 ( α 9 − β 9 ) ( ( α 1 0 − β 1 0 ) − 2 ( α 8 − β 8 ) ) × 3 3 = ( α + β ) ( α 9 − β 9 ) 3 ( α 1 0 − β 1 0 + α β ( α 8 − β 8 ) ) = ( α 1 0 − β 1 0 + α 9 β − α β 9 ) 3 ( α 1 0 − β 1 0 + α 9 β − α β 9 ) = 3
Now, we can find the remainder of [ 2 0 1 4 3 9 4 2 ] (Which I guessed using the note.)
Answer: 7 2 9
The fact that a(n)=alpha^n-beta^n implies that a difference equation can be formed.
The following difference equation is formed a(n+2)=6a(n+1)-2a(n).
Clearly a(0)=0.
As alpha=3+sqrt(11) and beta=3-sqrt(11), a(1)=2sqrt(11).
Therefore a(10) and a(8) and a(9) can be generated iteratively.
Giving a(10)=30489792sqrt(11), a(8)=19152sqrt(11), a(9)=4826912sqrt(11).
Therefore, cancelling the sqrt(11)'s, k=(30489792-2 19152)/(2 4826912)=3.
As 3 and 2014 and coprime, Euler's totient theorem can be used. phi(2014)=936.
Therefore k=3^6 mod(11). i.e. The remainder is 3^6=729.
(x – 3)^2 = 11
x = 3 +/- Sqrt (11)
p = 3 + Sqrt (11) and q = 3 - Sqrt (11)
a (n) = [3 + Sqrt (11)]^n – [3 - Sqrt (11)]^n for n >= 1
k = [a (10) – 2 a (8)]/ [2 a (9)] = 3
k^ 942 MOD 2014 = ?
3^942 MOD (2 x 19 x 53) = 729? {Calculator}
With 3^67 = 92709463147897837085761925410587
3^ [14 (67) + 4] MOD 2014
= (92709463147897837085761925410587^14 x 81)/ (2 x 19 x 53)
= (92709463147897837085761925410587 x
92709463147897837085761925410587^13 x 81)/ 2014
= (46032504045629511959166795139+641/ 2014) x 92709463147897837085761925410587^13 x 81
--> (641/ 2014) x 92709463147897837085761925410587^13 x 81
--> (641^14/ 2014) x 81
= (410881^7)/ 2014 x 81
--> (25^7)/ 2014 x 81
= 6103515625/ 2014 X 81
= 494384765625/ 2014
--> 729/ 2014
Remainder = 729
got 3 but stuck to find remainder for a while.
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Let α = c , β = d
c 2 − 6 c − 2 = 0
c 1 0 − 6 c 9 − 2 c 8 = 0
similarly,
d 1 0 − 6 d 9 − 2 d 8 = 0
Subtracting,
c 1 0 − d 1 0 − 6 ( c 9 − b 9 ) − 2 ( c 8 − d 8 ) = 0
a 1 0 − 2 a 8 = 6 a 9
2 a 9 a 1 0 − 2 a 8 = 3
3 9 4 2 = 2 0 1 4 k + 7 2 9
hence remainder is 729