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Which of the following numbers should be added to 5 44 + 6 5^{44}+6 to make the resultant number divisible by 8?

4 1 3 2

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4 solutions

Chew-Seong Cheong
Mar 18, 2017

5 44 + 6 ( 5 44 mod ϕ ( 8 ) + 6 ) (mod 8) Since gcd ( 5 , 8 ) = 1 , Euler’s theorem applies. ( 5 44 mod 4 + 6 ) (mod 8) Euler’s totient function ϕ ( 8 ) = 4 ( 5 0 + 6 ) (mod 8) 7 (mod 8) \begin{aligned} 5^{44}+6 & \equiv \left(5^{\color{#3D99F6} 44 \text{ mod }\phi (8)} + 6 \right) \text{ (mod 8)} & \small \color{#3D99F6} \text{Since }\gcd(5,8) = 1 \text{, Euler's theorem applies.} \\ & \equiv \left(5^{\color{#3D99F6} 44 \text{ mod }4} + 6 \right) \text{ (mod 8)} & \small \color{#3D99F6} \text{Euler's totient function }\phi (8) = 4 \\ & \equiv \left(5^{\color{#3D99F6}0} + 6 \right) \text{ (mod 8)} \\ & \equiv 7 \text{ (mod 8)} \end{aligned}

5 44 + 6 + 1 0 (mod 8) \implies 5^{44}+6 + \boxed{1} \equiv 0 \text{ (mod 8)} .

@Chew-Seong Cheong Sir since you use colors so well in your solution..

Can you help me out with this??

10 0 2 + 10 1 2 + 10 2 2 + 10 3 2 + 10 4 2 100^2 + 101^2 + 102^2 + \color{#3D99F6}{103^2} + 104^2

I just wanted to color 10 3 2 103^2 but this did not happen..Can you please patch up the hole?

Ankit Kumar Jain - 4 years, 2 months ago

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I got the error sir , thanks!!

Ankit Kumar Jain - 4 years, 2 months ago
Jesse Nieminen
Mar 18, 2017

5 44 + 6 ( 3 ) 44 + 6 3 44 + 6 9 22 + 6 1 22 + 6 1 + 6 7 1 ( m o d 8 ) 8 5 44 + 6 + 1 5^{44}+6 \equiv \left(-3\right)^{44}+6\equiv3^{44}+6\equiv9^{22}+6\equiv1^{22}+6\equiv1+6\equiv7\equiv-1\pmod{8}\implies8\mid5^{44}+6+1 .

Hence, the answer is 1 \boxed{1} .

Achal Jain
Mar 18, 2017

Note that 5 2 1 m o d 8 \large { 5 }^{ 2 }\equiv 1\quad mod\quad 8

The Congruence can be written as 1 + 6 m o d 8 \large 1+6\equiv \quad mod\quad 8

the sum is equivalent to 7 Hence 1 should be added \large \therefore \text{the sum is equivalent to} 7 \hspace{2cm} \text{Hence 1 should be added}

Kurupati Harsha
Apr 24, 2017

5^(even)gives you last digits as 625 where as 5^(odd) gives you 125.. as 5^44 is even power 625 are the last 3 digits which are needed to check if number is divisble by 8 .remainder is 7 so add 1.

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