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What is the sum of the tens and units digit of 9 2004 9^{2004} ?


The answer is 7.

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2 solutions

Harsh Khatri
Mar 3, 2016

9 2004 ( 9 ϕ ( 100 ) ) 50 9 4 1 6561 61 ( m o d 100 ) 9^{2004} \equiv (9^{\phi(100)})^{50}\cdot 9^4 \equiv 1 \cdot 6561 \equiv 61 \pmod{100}

Hence, the sum of the tens and units place is 6 + 1 = 7 6+1= \boxed{7}

As we can observe, it is can be expanded through Binomial Expansion so,

9 2004 9^{2004}

= = ( 10 1 ) 2004 (10-1)^{2004}

= = 1 0 2004 10^{2004} - ( 2004 1 ) \binom{2004}{1} 1 0 2003 10^{2003} + + . . . ... + + ( 2004 2002 ) \binom{2004}{2002} 1 0 2 10^2 - ( 2004 2003 ) \binom{2004}{2003} 10 10 + + 1 1

Using modulo 100,

-> - ( 2004 2003 ) \binom{2004}{2003} 10 10 + + 1 1 mod 100

-> \equiv 40 -40 + + 1 1 mod 100

-> \equiv 39 -39 mod 100

-> \equiv 61 61 mod 100

= = 6 + 1 6 + 1 = = 7 7

therefore, 7 is the sum.

You should change topic to number theory

Mohit Gupta - 5 years, 3 months ago

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Yes, sorry about that.

Russel Ryan Floresca - 5 years, 3 months ago

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