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# Prove that the last two digits are 28

Prove that if $$n$$ is an odd positive integer, then the last two digits of $$2^{2n}(2^{2n+1}−1)$$ in base $$10$$ are $$28$$.

Note by Finn Hulse
3 years, 5 months ago

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Hint:Induction

- 3 years, 5 months ago

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Okay... Can you explain?

- 3 years, 5 months ago

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When n=1, the result is 28.Suppose for some n the condition is true.

Multiply by 16^2 and add $$15.2^{2n+4}$$ and you will get the next such number.So the new number will be congruent to

$$256.28+15.2^{2n+4}\equiv 68+15.2^{2n+4}(mod100)$$

But $$15.2^{2n}$$ for odd numbers n will be congruent to 60 modulo 100 (that's not hard to see), so we have proven in inductively.

- 3 years, 5 months ago

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Oh, I guess so! Great job Bogdan. :D

- 3 years, 5 months ago

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You probably have no idea how to pronounce my name :D

- 3 years, 5 months ago

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Probably. How? :D

- 3 years, 5 months ago

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Try using the speech feature on it.It sounds stupid :D

- 3 years, 5 months ago

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HAHA yeah. But how is your name actually pronounced?

- 3 years, 5 months ago

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I believe it should be pronounced like this: pronounce log, but with a B instead of an l, then say the name Dan :D

- 3 years, 5 months ago

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Oh, that's what I thought. :D

- 3 years, 5 months ago

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