What is the remainder when 4 4 4 4 4 4 4 4 is divided by 9 ?
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Note: 4444's digit sum is 16, therefore 4444 mod 9 = 16 mod 9
Note: ϕ ( 9 ) = 9 × ( 1 − 3 1 ) = 6
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Could not understand this statement above, what's that?
Same :) & thus upvoted!
First note that 4 4 4 4 4 4 4 4 = ( 9 ∗ 4 9 3 + 7 ) 4 4 4 4 ≡ 7 4 4 4 4 ( m o d 9 ) .
Next, note that 7 3 = 3 4 3 = 9 ∗ 3 8 + 1 , and so 7 3 ≡ 1 ( m o d 9 ) .
Finally, note that 4 4 4 4 = 3 ∗ 1 4 8 1 + 1 , and so
4 4 4 4 4 4 4 4 ≡ 7 3 ∗ 1 4 8 1 + 1 ≡ 1 ∗ 7 ( m o d 9 ) ,
i.e., the desired remainder is 7 .
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From Euler's theorem, 4 4 4 4 4 4 4 4 m o d 9 = ( 4 4 4 4 m o d 9 ) 4 4 4 4 m o d ϕ ( 9 ) m o d 9 = ( 1 6 m o d 9 ) 4 4 4 4 m o d 6 m o d 9 = 7 4 m o d 9 = 4 9 2 m o d 9 = 4 2 m o d 9 = 1 6 m o d 9 = 7