What is the last digit of
1 7 1 8 2 × 8 4 3 3 + 2 3 7 9 × 1 1 9 4
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Relevant wiki: Euler's Theorem
Let N be the number given. We need to find N m o d 1 0 .
N ≡ 1 7 1 8 2 m o d ϕ ( 1 0 ) ( 2 3 ) 4 3 3 + 2 3 7 9 m o d ϕ ( 1 0 ) ( 1 0 + 1 ) 9 6 (mod 10) ≡ 1 7 1 8 2 m o d 4 ( 2 4 × 3 2 4 + 3 ) + 2 3 7 9 m o d 4 ( 1 ) (mod 10) ≡ 1 7 2 ( 1 6 3 2 4 × 8 ) + 2 3 3 (mod 10) ≡ 7 2 × 6 × 8 + 3 3 (mod 10) ≡ 9 × 8 + 7 (mod 10) ≡ 2 + 7 (mod 10) ≡ 9 (mod 10) Since g cd ( 1 7 , 1 0 ) = g cd ( 2 3 , 1 0 ) = 1 , Euler’s theorem applies. Euler’s totient function ϕ ( 1 0 ) = 4 Power of integer ends with 6 always ends with 6.
Considering the first term: 1 7 1 8 2
7 1 = 7 , 7 2 = 4 9 , 7 3 = 3 4 3 , 7 4 = 2 4 0 1 , 7 5 = 1 6 8 0 7 observed that the units digit repeat in every cycle of 4 . So we divide the exponent by 4 then find the remainder.
4 1 8 2 = 4 5 4 2 : the remainder is 2 so the last digit is 9 .
Considering the second term: 8 4 3 3
8 1 = 8 , 8 2 = 6 4 , 8 3 = 5 1 2 , 8 4 = 4 0 9 6 , 8 5 = 3 2 7 6 8 observed that the units digit repeat in every cycle of 4. So we divide the exponent by 4 then find the remainder.
4 4 3 3 = 1 0 8 4 1 : the remainder is 1 so the last digit is 8
Considering the third term: 2 3 7 9
3 1 = 3 , 3 2 = 9 , 3 3 = 2 7 , 3 4 = 8 1 , 3 5 = 3 4 3 observed that the units digit repeat in every cycle of 4. So we divide the exponent by 4 then find the remainder.
4 7 9 = 1 9 4 3 : the remainder is 3 so the last digit is 7 .
Considering the fourth term: 1 1 9 4
1 1 = 1 , 1 2 = 1 , 1 3 = 1 observed that the last digit is always 1 so the last digit of the fourth term is 1 .
The last digit of the first two terms is 2 ( 9 × 8 = 7 2 ) .
The last digit of the last two terms is 7 ( 7 × 1 = 7 ) .
Adding the two gives
2 + 7 = 9
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Let A be the number given.
A = 1 7 1 8 2 × 8 4 3 3 + 2 3 7 9 × 1 1 9 4 = ( 1 7 4 ) 4 5 × 1 7 2 × ( 2 1 0 ) 1 2 9 × 2 9 + ( 2 3 4 ) 1 9 × 2 3 3 × 1 1 9 4 = ( 8 3 5 2 1 ) 4 5 × 2 8 9 × ( 1 0 2 4 ) 1 2 9 × ( 2 ) 9 + ( 2 7 9 8 4 1 ) 1 9 × ( 2 3 ) 3 × ( 1 1 ) 9 4 = 1 × 9 × 4 × 2 + 1 × 7 × 1 = 2 + 7 = 9