Remainder

What is the remainder when ( 2 2014 + 7 × 5 4020 ) (2^{2014}+7×5^{4020}) is divided by 23?


The answer is 0.

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2 solutions

Anik Mandal
Sep 10, 2014

5 2 2 ( m o d 23 ) 5^{2}\equiv{2}\pmod{23}

5 4020 2 2010 ( m o d 23 ) 5^{4020}\equiv{2^{2010}}\pmod{23}

Hence 5 4020 5^{4020} and 2 2010 2^{2010} both leave the same reminder when divided by 23 23 .

So asked thing is reduced to finding 2 2014 + 7 2 2010 ( m o d 23 ) 2^{2014}+7\cdot2^{2010}\pmod{23}

2 2014 + 7. 2 2010 2^{2014}+7.2^{2010}

= 2 2010 ( 2 4 + 7 ) 2^{2010}(2^{4}+7)

= 2 2010 . 23 2^{2010}.23

Hence.the expression is divisible by 23 23 and leaves a remainder of 0 0 when divided by 23 23 .

@Anik Mandal , next time, for ( m o d a ) \pmod{a} notation, use the LaTeX \LaTeX code, you may want to see it, i've edited that in your solution.

Aditya Raut - 6 years, 9 months ago
Aditya Raut
Sep 10, 2014

By Fermat's little theorem , as 23 is a prime,

2 22 1 ( m o d 23 ) 2 22 × 91 = 2 2002 1 ( m o d 23 ) 2 2014 2 12 ( m o d 23 ) 2^{22} \equiv 1 \pmod{23}\\ \therefore 2^{22\times 91}=2^{2002} \equiv 1 \pmod{23} \\ \therefore 2^{2014} \equiv 2^{12} \pmod{23}


Similarly doing for the second term, we have

5 4004 1 ( m o d 23 ) 5 4020 5 16 ( m o d 23 ) 5^{4004}\equiv 1 \pmod{23} \\ \therefore 5^{4020} \equiv 5^{16} \pmod{23}


Moreover, we have

5 2 25 2 ( m o d 23 ) 5 16 2 8 ( m o d 23 ) 5^2 \equiv 25\equiv 2\pmod{23} \\ \implies 5^{16} \equiv 2^8 \pmod{23}


So asked thing is reduced to finding 2 12 + 7 × 2 8 ( m o d 23 ) 2^{12} + 7\times 2^{8} \pmod{23}

This is simple by following way, 2 12 + 7 × 2 8 2 8 ( 2 4 + 7 ) 2 8 ( 23 ) 0 ( m o d 23 ) 2^{12} + 7\times 2^8\equiv 2^8 (2^4+7) \equiv 2^8(23) \equiv \boxed{0}\pmod{23}

Great use of FLT!

Krishna Ar - 6 years, 9 months ago

I too did the same way ! ! !!

Mehul Chaturvedi - 6 years, 5 months ago

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