What is the last 4 digits of 2 2 0 1 6 ?
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You should mention that the 2 4 x ≡ 1 ( m o d 5 ) can be proven using Fermat's Little Theorem.
To get the last 4 digits of 2 2 0 1 6 , we need to get the remainder when it is divided by 10000.
To get the remainder you need to use CRT or Modular Arithmetic
By Modular Arithmetic:
2^2016mod10000=2^8^252mod10000=256^252mod10000=65536^126mod10000=5536^126mod10000=30647296^63mod10000=7296^63mod10000=(1616^31)(7296)mod10000=(1456^15)(1616)(7296)mod10000=(9936^7)(1456)(1616)(7296)mod10000=(4096^3)(9936)(7296)(2896)mod10000=(7216)(7856)(9216)mod10000=(7216)(896)mod10000=5536mod10000
Therefore, the last 4 digits of 2 2 0 1 6 is 5 5 3 6
Even though you got it right, it's better to do as little calculation as possible.
Can we use this to find last digit of any exponent ?
If yes then good method
Yes we can. It's simply m o d 1 0 .
we need to divide by 10,000 and remainder would be last 4 digits. 2^2016=65536^126= (6*10000+5536)^126 = 126C0(10000)126+... last term of expansion would be 5536. Each term has atleast one 10,000. Hence remainder is 5536
This solution has been marked wrong. The last term of the expansion is 5 5 3 6 1 2 6 , you still got quite a number of manual labor to go through this. And it is impractical to calculate or memorize 2 1 6 = 6 5 5 3 6 .
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Firstly, 2 4 x ≡ 1 ( m o d 5 ) with every positive integer x (easily proved).
As a result:
2 2 0 − 1 = ( 2 4 − 1 ) ( 2 1 6 + 2 1 2 + 2 8 + 2 4 + 1 ) is divisible by 25, means 2 2 0 ≡ 1 ( m o d 2 5 ) ;
2 1 0 0 − 1 = ( 2 2 0 − 1 ) ( 2 8 0 + 2 6 0 + 2 4 0 + 2 2 0 + 1 ) is divisible by 125, means 2 1 0 0 ≡ 1 ( m o d 1 2 5 )
Once more and we have: 2 5 0 0 ≡ 1 ( m o d 6 2 5 )
So: 2 2 0 0 0 ≡ 1 ( m o d 6 2 5 )
( 2 2 0 0 0 − 1 ) is divisible by 625 -> ( 2 2 0 0 4 − 2 4 ) is divisible by 10000
Then ( 2 2 0 1 6 − 2 1 6 ) is divisible by 10000
The last 4 digits of 2 2 0 1 6 is the same as that of 2 1 6 , which is 5 5 3 6 .