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Calculate remainder of $12^{34^{56^{78}}}$ when divided by $90$

First of all,I'm not sure about the order of calculation of powers!
Secondly I don't know the rules of finding remainder moulus 90

5 Answers 5

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My solution is rather routine and makes good use of Euler totient theorem and modulus properties.

$$\begin{align*}12^{34^{45^{78}}} \pmod{90} &\equiv 6\left[\frac{12^{34^{45^{78}}}}{6} \pmod{15} \right],\qquad\qquad\gcd(12,90)=6\\ &\equiv 18\left[\frac{12^{34^{45^{78}}}}{18} \pmod{5}\right],\qquad\qquad\gcd(12,15)=3\\ &\equiv 18\left[\frac{2^{34^{45^{78}}\pmod 4}}{3} \pmod{5}\right],\qquad\qquad\phi(5)=5-1=4\\ &\equiv 18\left[\frac{2^0}{3} \pmod{5}\right]\\ &\equiv 18\left[\frac{6}{3} \pmod{5}\right]\\ &\equiv 18\left[2\right]\equiv 36 \end{align*}$$

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Hint $\,\ N>0\,\Rightarrow\,18\mid 12^{4N}\!\Rightarrow 12^{4N}\!\bmod{90} \,=\,18\underbrace{\left(\dfrac{(12^{4})^N}{18}\bmod 5\right)}_{\equiv\ \dfrac{1^N}{3}\ \equiv\ \dfrac{6}3\equiv\ \Large\color{#c00} 2} =\, 18(\color{#c00}2)\ $

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    We used the [mod Distributive Law](http://math.stackexchange.com/a/2059937/242) $\ ca\bmod cn = c(a\bmod n)\ \ $ – 2017-01-30
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    @Downvoter If something is not clear then please feel welcome to ask questions and I will be happy to elaborate. – 2017-02-15
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$90=2 \times 3^2 \times 5$, so let us divide the cases. First note $$12^{34^{56^{78}}} \equiv 0 \pmod {2}$$$$12^{34^{56^{78}}}=4^{34^{56^{78}}} \times 3^{34^{56^{78}}} \equiv 0 \pmod {3^2}$$ Also, since $56^{78}>2$, $34^{56^{78}} \equiv 0 \pmod {4}$. Let $34^{56^{78}} =4k$ where $k$ is a natural. So $$12^{34^{56^{78}}}=12^{4k}=(12^{4})^k \equiv 1^k \equiv 1 \pmod {5}$$ From Fermat's Little Theorem. Now use CRT. So we have that $$12^{34^{56^{78}}} \equiv 36 \pmod {90}$$

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    What about the order of power calculation?? – 2017-01-29
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    @HamidRezaEbrahimi If you see my answer, you can see the order in which he calculate powers. First calculate $56^{78}$. Then calculate $34^{56^{78}}$. Then calculate $12^{34^{56^{78}}}$. – 2017-01-29
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    Start from the exponents downwards: $(12)^{34^{56^{78}}} = (12)^{34^{(2.28\cdot 10^{136})}} = ...$ etc – 2017-01-29
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    Isn't CRT helpful further, by this 3 must be remainer – 2017-01-29
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    @SubhashChandBhoria What do you mean? – 2017-01-29
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    @S.C.B. your solution seems fine just CRT will give further remainder as 3 – 2017-01-29
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    @HamidRezaEbrahimi Do you understand? – 2017-01-29
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    @S.C.B. Yes, thanks. – 2017-01-29
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    I have already upvoted. Just waiting for a reasonable amount of time,don't hurry up – 2017-01-29
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    Why the downvote? – 2017-02-15
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    @S.C.B. Curiously, my answer was downvoted 37 seconds after you posted the above comment in this old, inactive thread. Explanation? $\ \ $ – 2017-02-15
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    @BillDubuque I will upvote then. – 2017-02-16
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    @S.C.B. Thanks, but it is not so much votes that I care about but, rather, understanding what might be perceived as deficient, so that I can improve the exposition. – 2017-02-16
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As $34^{56^{78}}$ is divisible by $4,$ let us write this as $4(m+1)$

As $(12^n,90)=18$ for $n\ge2$ let us find $12^{4(n+1)-2}\pmod{90/18}$

i.e., $12^{4n+2}\pmod5$

Now as $12\equiv2\pmod5,12^2\equiv2^2\equiv-1$

$12^{4m+2}=(12^2)^{2m+1}\equiv(-1)^{2m+1}\equiv-1\pmod5$

$\implies12^{4(m+1)}\equiv12^2\cdot-1\pmod{12^2\cdot5}\equiv-144\pmod{90}$ as $90|12^2\cdot5$

Finally $-144\equiv36\pmod{90}$ as $12^{4(m+1)}>0$

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    See also : http://math.stackexchange.com/questions/1385396/last-two-digits-of-145532/1386230#1386230 OR http://math.stackexchange.com/questions/808747/what-is-the-remainder-when-241202-is-divided-by-1446/808751#808751 OR http://math.stackexchange.com/questions/1952810/finding-the-last-two-digits-of-5312442 OR http://math.stackexchange.com/questions/1844558/how-to-find-last-two-digits-of-22016 – 2017-01-31
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Working through this slowly:

For the sequence $12^k \bmod 90$, we can see this will quickly settle onto multiples of $18$, since $12$ and $90$ share $2$ and $3$ as prime factors and $2\cdot 3^2$ is the largest factor of $90$ using these prime factors.

So the sequence $12^k \bmod 90$ looks like $\{12, \color{blue}{54}, 18, 36, 72, \color{blue}{54}, \ldots\}$ with a cycle length of $4$, and thus $12^{4i}\equiv 36 \bmod 90$ for $i\ge 1$.

Then for $j>1$, $4 \mid 34^j$ and thus $12^{34^{56^{78}}}\equiv 36 \bmod 90$

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    Nice and concise – 2017-01-31