Find the ones/units digit

What is the last digit of

1 7 182 × 8 433 + 2 3 79 × 1 1 94 \large{17^{182} \times 8^{433} + 23^{79} \times 11^{94}}


The answer is 9.

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3 solutions

Let A A be the number given.

A = 1 7 182 × 8 433 + 2 3 79 × 1 1 94 = ( 1 7 4 ) 45 × 1 7 2 × ( 2 10 ) 129 × 2 9 + ( 2 3 4 ) 19 × 2 3 3 × 1 1 94 = ( 83521 ) 45 × 289 × ( 1024 ) 129 × ( 2 ) 9 + ( 279841 ) 19 × ( 23 ) 3 × ( 11 ) 94 = 1 × 9 × 4 × 2 + 1 × 7 × 1 = 2 + 7 = 9 \begin{aligned} A &= 17^{182}\times8^{433}+23^{79}\times11^{94} \\ & =(17^{4})^{45}\times17^{2}\times(2^{10})^{129}\times2^{9} + (23^{4})^{19}\times23^{3}\times11^{94} \\ & = (83521)^{45}\times289\times(1024)^{129}\times(2)^{9} + (279841)^{19}\times(23)^{3}\times(11)^{94} \\ & = 1\times9\times4\times2 + 1\times7\times1 \\ & = 2+ 7 = \boxed{9} \end{aligned}

Chew-Seong Cheong
Mar 29, 2018

Relevant wiki: Euler's Theorem

Let N N be the number given. We need to find N m o d 10 N \bmod 10 .

N 1 7 182 m o d ϕ ( 10 ) ( 2 3 ) 433 + 2 3 79 m o d ϕ ( 10 ) ( 10 + 1 ) 96 (mod 10) Since gcd ( 17 , 10 ) = gcd ( 23 , 10 ) = 1 , Euler’s theorem applies. 1 7 182 m o d 4 ( 2 4 × 324 + 3 ) + 2 3 79 m o d 4 ( 1 ) (mod 10) Euler’s totient function ϕ ( 10 ) = 4 1 7 2 ( 1 6 324 × 8 ) + 2 3 3 (mod 10) Power of integer ends with 6 always ends with 6. 7 2 × 6 × 8 + 3 3 (mod 10) 9 × 8 + 7 (mod 10) 2 + 7 (mod 10) 9 (mod 10) \begin{aligned} N & \equiv 17^{\color{#3D99F6}182 \bmod \phi(10)} \left(2^3\right)^{433} + 23^{\color{#3D99F6}79 \bmod \phi(10)}(10+1)^{96} \text{ (mod 10)} &\small \color{#3D99F6} \text{Since }\gcd (17,10) = \gcd(23, 10) = 1 \text{, Euler's theorem applies.} \\ & \equiv 17^{\color{#3D99F6}182 \bmod 4} \left(2^{4 \times 324+3}\right) + 23^{\color{#3D99F6}79 \bmod 4}(1) \text{ (mod 10)} & \small \color{#3D99F6} \text{Euler's totient function }\phi(10) = 4 \\ & \equiv 17^{\color{#3D99F6}2} \left({\color{#D61F06}16^{324}}\times 8\right) + 23^{\color{#3D99F6}3} \text{ (mod 10)} & \small \color{#D61F06} \text{Power of integer ends with 6 always ends with 6.} \\ & \equiv 7^2\times {\color{#D61F06}6} \times 8 + 3^3 \text{ (mod 10)} \\ & \equiv 9 \times 8 + 7 \text{ (mod 10)} \\ & \equiv 2+7 \text{ (mod 10)} \\ & \equiv \boxed{9} \text{ (mod 10)} \end{aligned}

Considering the first term: 1 7 182 17^{182}

7 1 = 7 , 7 2 = 49 , 7 3 = 343 , 7 4 = 2401 , 7 5 = 16807 7^1=7,7^2=49,7^3=343,7^4=2401,7^5=16807 observed that the units digit repeat in every cycle of 4 4 . So we divide the exponent by 4 4 then find the remainder.

182 4 = 45 2 4 \dfrac{182}{4}=45 \dfrac{2}{4} : the remainder is 2 2 so the last digit is 9 9 .

Considering the second term: 8 433 8^{433}

8 1 = 8 , 8 2 = 64 , 8 3 = 512 , 8 4 = 4096 , 8 5 = 32768 8^1=8, 8^2=64, 8^3=512, 8^4=4096, 8^5=32768 observed that the units digit repeat in every cycle of 4. So we divide the exponent by 4 4 then find the remainder.

433 4 = 108 1 4 \dfrac{433}{4}=108 \dfrac{1}{4} : the remainder is 1 1 so the last digit is 8 8

Considering the third term: 2 3 79 23^{79}

3 1 = 3 , 3 2 = 9 , 3 3 = 27 , 3 4 = 81 , 3 5 = 343 3^1=3,3^2=9,3^3=27,3^4=81,3^5=343 observed that the units digit repeat in every cycle of 4. So we divide the exponent by 4 4 then find the remainder.

79 4 = 19 3 4 \dfrac{79}{4}=19 \dfrac{3}{4} : the remainder is 3 3 so the last digit is 7 7 .

Considering the fourth term: 1 1 94 11^{94}

1 1 = 1 , 1 2 = 1 , 1 3 = 1 1^1=1,1^2=1,1^3=1 observed that the last digit is always 1 1 so the last digit of the fourth term is 1 1 .

The last digit of the first two terms is 2 2 ( 9 × 8 = 72 ) (9 \times 8=72) .

The last digit of the last two terms is 7 7 ( 7 × 1 = 7 ) (7 \times 1 = 7) .

Adding the two gives

2 + 7 = 9 2+7=\boxed{9}

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