More odd patterns and even too!

Number Theory Level pending

What is the last digit of: 2 2 × 3 3 × 4 4 × 6 6 × 7 7 × 8 8 × 9 9 × 1 2 12 × 1 3 13 × 1 4 14 × 1 6 16 × 1 7 17 × 1 8 18 × 1 9 19 × × 9 9 99 \begin{array} {llllllll} & 2^{2}& \times3^{3}& \times4^{4}& \times6^{6}& \times7^{7}& \times8^{8}& \times9^{9}\\ \times & 12^{12} & \times13^{13} &\times14^{14} &\times16^{16} &\times17^{17} &\times18^{18} &\times19^{19} \\ \times & \ldots & & & & & &\times99^{99} \end{array}

6 4 2 8

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2 solutions

Otto Bretscher
Sep 14, 2015

Working modulo 5 (and modulo ϕ ( 5 ) = 4 \phi(5)=4 in the exponents), the given number n n reduces to 2 2 3 3 2 3 4 3 4 2 2 = 2 12 3 4 1 ( m o d 5 ) 2^2*3^3*2^3*4*3*4^2*2=2^{12}*3^4\equiv{1}\pmod{5} . Since n n is even, the last digit must be 6 \boxed{6}

Hm, can you review this solution? I'm guessing that you didn't see the full extent of the problem (which I have now edited for clarity).

Calvin Lin Staff - 5 years, 9 months ago

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Oops...it looked like the product was ending at 1 7 17 17^{17} . Actually, since ( n + 20 ) n + 20 n n ( m o d 10 ) (n+20)^{n+20}\equiv{n^n}\pmod{10} , I was not that far off. I was just "lucky" that 1 8 18 1 9 19 1 ( m o d 5 ) 18^{18}*19^{19}\equiv{1}\pmod{5} .

Otto Bretscher - 5 years, 9 months ago
Rob Matuschek
Sep 14, 2015

the last digit will be modeled by the last digit of 2 n 2 × 3 n 3 × 4 n 4 × 6 n 6 × 7 n 7 × 8 n 8 × 9 n 9 2^{n2}\times3^{n3}\times4^{n4}\times6^{n6}\times7^{n7}\times8^{n8}\times9^{n9} where n 3 n3 and n 7 n7 are divisible by 4, and n 2 n2 and n 8 n8 are even but not divisible by 4, and n 4 n4 and n 9 n9 are even, thus the last digit of the above product reduces to 4 × 1 × 6 × 6 × 1 × 4 × 1 = 6. 4\times1\times6\times6\times1\times4\times1=6. The cyclical patterns of the last digit of the powers of each digit are as follows: 2 > ( 2 , 4 , 8 , 6 ) 2->(2,4,8,6) , 3 > ( 3 , 9 , 7 , 1 ) 3->(3,9,7,1) , 4 > ( 4 , 6 ) 4->(4,6) , 5 > ( 5 ) 5->(5) , 6 > ( 6 ) 6->(6) , 7 > ( 7 , 9 , 3 , 1 ) 7->(7,9,3,1) , 8 > ( 8 , 4 , 2 , 6 ) 8->(8,4,2,6) , 9 > ( 9 , 1 ) 9->(9,1)

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