Find the remainder.

Find the remainder when 5 2009 + 1 3 2009 5^{2009} + 13^{2009} is divided by 18.

2 1 None of these 0

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2 solutions

Ryan Tamburrino
Jan 22, 2015

We can quickly deduce that 1 3 2009 ( 5 ) 2009 m o d 18 13^{2009} \equiv (-5)^{2009} \mod{18} and ( 5 ) 2009 = ( 5 2009 ) (-5)^{2009} = -(5^{2009})

Substituting, we find that 5 2009 + 1 3 2009 5 2009 5 2009 = 0 m o d 18 5^{2009} + 13^{2009} \equiv 5^{2009} - 5^{2009} = \boxed{0} \mod{18}

Mehul Chaturvedi
Jan 21, 2015

We know

For any positive integer a , b , n , e a,b,n,e

a n + b n m o d e = ( a + c ) 9 m o d e a^n+b^n \mod e=(a+c)^9 \mod e

5 2009 + 1 3 2009 m o d 18 = ( 5 + 13 ) 2009 m o d e \therefore 5^{2009}+13^{2009} \mod 18=(5+13)^{2009} \mod e

and we know if a = b m o d e a=b\mod e then a n = b n m o d e a^n=b^n\mod e

( 5 + 13 ) 2009 m o d 18 = 0 2009 m o d 18 \therefore (5+13)^{2009} \mod 18=0^{2009} \mod 18

hence our answer is 0 \boxed{0}

In the first step, do you mean a n + b n ( m o d e ) = ( a + c ) n ( m o d e ) a^{n}+b^{n} \pmod{e}=(a+c)^{n}\pmod{e} ? If you meant for it to be 9 9 , then why?

Omkar Kulkarni - 6 years, 4 months ago

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