Find the remainder when 5 2 0 0 9 + 1 3 2 0 0 9 is divided by 18.
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We know
For any positive integer a , b , n , e
a n + b n m o d e = ( a + c ) 9 m o d e
∴ 5 2 0 0 9 + 1 3 2 0 0 9 m o d 1 8 = ( 5 + 1 3 ) 2 0 0 9 m o d e
and we know if a = b m o d e then a n = b n m o d e
∴ ( 5 + 1 3 ) 2 0 0 9 m o d 1 8 = 0 2 0 0 9 m o d 1 8
hence our answer is 0
In the first step, do you mean a n + b n ( m o d e ) = ( a + c ) n ( m o d e ) ? If you meant for it to be 9 , then why?
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We can quickly deduce that 1 3 2 0 0 9 ≡ ( − 5 ) 2 0 0 9 m o d 1 8 and ( − 5 ) 2 0 0 9 = − ( 5 2 0 0 9 )
Substituting, we find that 5 2 0 0 9 + 1 3 2 0 0 9 ≡ 5 2 0 0 9 − 5 2 0 0 9 = 0 m o d 1 8