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After simplifying $\frac{2^{2017}+1}{3\cdot 2^{2017}}$ to $\frac{n}{m}$ where $n$ and $m$ are coprime, find the remainder when $m+n$ is divided by $1000$.

Since the top is odd and the bottom is even, the only thing that can cancel is the $3$. Therefore, I'm looking for $2^{2017}+\frac{2^{2017}+1}{3}$ (mod $1000$). However, I don't know how to find this value. Could someone give me a hint?

nonuser
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Silverleaf1
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    Good start. To finish, note that $3n\equiv 1 \pmod {1000}\implies n\equiv 667\pmod {1000}$, so you just want $(2^{2017}+1)\times 667\pmod {1000}$ (for the second term). – lulu Feb 09 '20 at 18:04
  • but how is $(2^{2017}+1)*667$ (mod $1000$) any easier to solve? am I missing something obvious? – Silverleaf1 Feb 09 '20 at 18:09
  • Well, $2^{2017}\pmod {1000}$ can be computed by iterated squaring, not too bad. You need that anyway, for the first term. – lulu Feb 09 '20 at 18:13
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    Even easier: it is easy to see that $2^{2017}\equiv 72 \pmod {5^3}$ which quickly shows that $2^{2017}\equiv 72\pmod {1000}$. – lulu Feb 09 '20 at 18:22

3 Answers3

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Clearly $\gcd (2^{2017}+1,2^{2017}3)=3$ so $$3m+3n =2^{2017}3+(2^{2017}+1)=2 ^{2019}+1$$

So $$3(m+n)\equiv _8 1$$ so $$m+n\equiv _8 3$$ and since $\varphi(125)=100$ we have by Euler theorem $$3(m+n)\equiv _{125} 2^{19}+1 = 2^9\cdot 2^{10}+1 \equiv _{125}39$$ $$ \implies m+n \equiv_{125} 42\cdot 39 \equiv_{125} 13$$

Write $m+n = 8x+3$ then we have $$125\mid 8x-10\implies 125\mid 4x-5 $$

$$\implies 125\mid 4x+120 = 4(x+30)\implies 125\mid x+30$$ so $x = 125y-30$ and thus $$ m+n = 1000y-237$$

So the answer is $\boxed{763}$.

nonuser
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We can write $3m=2^{2017}+1$ and $3n=3\cdot2^{2017}$

Now $3(m+n)=1+2^{2017}(1+3)=1+2^{2019}$

Now as $\phi(125)=100$ and $(2,125)=1$

Like How to find last two digits of $2^{2016}$

$2^{2016}\equiv2^{16}\equiv(2^8)^2\equiv6^2\pmod{125}$

$\implies2^{2016+3}\equiv2^36^2\pmod{125\cdot2^3}$

$\implies3(m+n)\equiv1+36\cdot8\pmod{1000}$

$\implies m+n\equiv289\cdot3^{-1}\equiv(96\cdot3+1)(3^{-1})\equiv96-333+1000$

as $3^{-1}\equiv1\equiv-333\equiv-333+1000$

lab bhattacharjee
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$3*2^{2017}$ has only prime factors of $2$ and $3$ an $2^{2017} + 1$ is not divisible by $2$ so the only thing that can cancel out is $3$. But does $3$ cancel out?

$2 \equiv -1\pmod 3$ so $2^{2017} + 1\equiv (-1)^{2017}+ 1\equiv -1 + 1 \equiv 0\pmod 3$ so, yes, $3$ cancels out.

So $\frac mn = \frac {2^{2017} + 1}{3*2^{2017}}$ so $m = \frac {2^{2017}+1}3$ and $n = 2^{2017}$.

So we need to find $m + n\pmod{1000}$.

$1000 = 8*125$ and by $n = 2^{2017}=8*2^{2014}\equiv 0 \pmod 8$ and be Euler's Th: as $\phi(125) = \phi(5^3)= 4*5^2= 100$ we have $2^{2017}\equiv 2^{17}\pmod {125}$.

Now $3$ is relatively prime to $8$ and to $125$ so $3^{-1}\mod 8$ and $3^{-1}\mod 125$ exist. And $m = (2^{2017}+1)*3^{-1} \equiv 3^{-1}\pmod 8$ and $m\equiv (2^{2017}+1)3^{-1}\equiv (2^{17}+1)*3^{-1}\pmod {125}$.

So $m + n \equiv 3^{-1} \pmod 8$ and $m +n \equiv 2^{17} + (2^{17}*+1)*3^{-1} \pmod {125}$.

.....

Deep breath

.....

so we need to figure out $2^{17}\pmod{125}$ and $3^{-1}\pmod {125}$ and $3^{-1}\pmod 8$.

...

$1\equiv 9=3*3\pmod 8$ so $3^{-1}\equiv 3\pmod 8$ and $m+n\equiv 3\pmod 8$.

$2^7= 128 \equiv 3\pmod {125}$ and $2^{10}\equiv 1024\equiv 24\pmod {125}$ so $2^{17} \equiv 72\pmod {125}$.

And $1\equiv 126= 3*42\pmod {125}$ so $3^{-1}\equiv 42\pmod {125}$.

So $m+n \equiv 72 + (72+1)*42\equiv 3138\equiv 13\pmod{125}$.

....

So $m+n \equiv 3\pmod 8$ and $m+n \equiv 13\pmod {125}$.

Now we need

$3 + 8m = 13 + 125k$

$8m = 10 + (8*15 + 5)k$

$8(m-15k) = 10 + 5k$

$8(m-15k) = 5(k+2)$

$m-15k = k+2 = 0$ will do.

$k=-2$ and $m=-30$ or

$3-8*30 = -237$ and $13-125*2=-237\equiv 763\pmod{1000}$

So I get $763$

fleablood
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