Last Digits

Find the last two digits of 2 2012 2^{2012} .


The answer is 96.

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2 solutions

A short outline of the solution...

Note

To find last two digits , we always take m o d 100 \mod 100

Using the Euler's Theorem , which is , a ϕ ( n ) 1 m o d n a^{\phi (n)} \equiv 1 \mod n . Applying it in the problem ...we have , ϕ ( 100 ) = 40 \phi (100) = 40 . So we arrive at a congruence which is 2 12 m o d 100 2^{12} \mod 100 . If you are wondering how that 12 came to the power of base 2 ,

then note that 12 is the remainder when we divide 2012 by 40.

So the final congruence is , 4096 m o d 100 4096 \mod 100 and hence the last two digits are 96

Thanks to Euler , who made it all easy...

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Ashwin K
Mar 2, 2016

Last 2 digits for powers of 2 cycles for every 20 and so we can find the last 2 digit directly for 2 12 2^{12} = 1024 * 4 =96

yes I did it the same way using the fact that the last two digits of powers of two reappear every 20 powers.

Sathvik Acharya - 4 years, 3 months ago

Oh thanks so much! I've never learned about this cycle before!

Đạt Tạ Quang - 4 years, 2 months ago

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