Last Two digits!

What is the last two digits of: 1 1 2018 ? \large 11^{2018}?


The answer is 81.

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4 solutions

Chris H
Jun 18, 2018

Pascal’s triangle has a property where if all the digits in the n n th row are read, it gives the n n th power of 11. ( 2018 2018 ) = 1 {2018 \choose 2018}=1 , giving the last digit. ( 2018 2017 ) = ( 2018 1 ) = 2018 {2018 \choose 2017}={2018 \choose 1}=2018 , so the last digits are ...20181. This leaves 81 as the last two digits.

Chew-Seong Cheong
Jun 19, 2018

Let N = 1 1 2018 N=11^{2018} . We need to find N m o d 100 N \bmod 100 .

N 1 1 2018 (mod 100) ( 10 + 1 ) 2018 (mod 100) ( 1 0 2018 + 2018 × 1 0 2017 + + 2018 × 2017 2 × 1 0 2 + 2018 × 10 + 1 ) (mod 100) 81 (mod 100) \begin{aligned} N & \equiv 11^{2018} \text{ (mod 100)} \\ & \equiv (10+1)^{2018} \text{ (mod 100)} \\ & \equiv \left(10^{2018} + 2018 \times 10^{2017} + \cdots + \frac {2018\times 2017}2 \times10^2 + 2018\times 10 + 1\right) \text{ (mod 100)} \\ & \equiv \boxed{81} \text{ (mod 100)} \end{aligned}

Hana Wehbi
Jun 17, 2018

Notice that:

1 1 2 m o d 100 = 21 11^2 \mod 100 =21 ,

1 1 3 m o d 100 = 31 11^3 \mod 100 =31 ,

1 1 4 m o d 100 = 41 etc 11^4\mod 100=41\dots \text { etc }

1 1 2018 = 1 1 2010 + 8 m o d 100 = ( 1 ) ( 81 ) = 81 m o d 100 the last two digits are 81. \implies 11^{2018} = 11^{2010+8} \mod 100 = (1)(81)= 81 \mod 100 \implies \text { the last two digits are 81. }

. .
Mar 26, 2021

1 1 2018 = 1 8 11 ^ { 2018 } = 1 ^ { 8 } when we look for the last digit, so last digit must be 1 1 .

Then we only need to look for the last-second digits.

It is 8 8 .

So, it is 81 \boxed { 81 } .

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