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Ten words (Just for fun)

How well can we summarise proofs?

Come up with proofs for the following statements in ten words or less. Who can make the most rigorous, accurate proof in just ten words?

\(1\). Among \(51\) integers from \(1\) to \(100 \) inclusive, there exists two summing to \(101\).


\(2\). \(8^{n}-5^{n}\) is a multiple of \(3\) where \(n\) is a positive integer.


\(3\). The point \(O\) in equilateral triangle \(ABC\) such that \(OA+OB+OC\) is minimised is the center of \(ABC\).

All operations, except for brackets, are words.

Note by Joel Tan
2 years, 6 months ago

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1) There are \(50\) pairs with sum \(101\). PHP. Mursalin Habib · 2 years, 6 months ago

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2) \[8^n - 5^n \equiv 2^n - 2^n = 0 \pmod{3}\] Sharky Kesa · 2 years, 6 months ago

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@Sharky Kesa Isn't it supposed to be \(8^{n}-5^{n}\) in the very first step...?? Rohit Sachdeva · 2 years, 6 months ago

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@Rohit Sachdeva Yes, typoed it. Sharky Kesa · 2 years, 6 months ago

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Proofs:

1) Gauss' pairing trick paired with pigeonhole principle.

2) \(=\) GP with ends \(5^{n-1}, 8^{n-1}\), ratio \(1.6\), summed thrice.

3) By construction of Fermat point, \(O = X(13)\).

Comments:

1) Alliteration 'p's yay! No I really did not want to refer to any language stuff.

2) It is trivial to see that all terms of the GP are integers, and ends refer to initial and final terms. I would have preferred \(3\sum^{n-1}_{k=0}8^{n-1-k}5^k\) but I am not sure how many words would that be.

3) Sorry for the overkill :P But anyway I could have mentioned an equilateral triangle is acute for clarity.

By the way, nice choice of problems! Yong See Foo · 2 years, 6 months ago

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2) If N>=0, 8^N - 5^N = 0 (mod 3) Ian Hoolihan · 2 years, 5 months ago

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There can be 49 pairs summing to 101 Gunit Varshney · 2 years, 6 months ago

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Q.2 It has one factor(8-5) = 3 means it is divisible by 3 Q.3 O is crcumcentre Ravibhole Bhole · 2 years, 6 months ago

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@Ravibhole Bhole Maximum is at the fermat point, not at the circumcentre, although it is true in this case since it is equilateral. Joel Tan · 2 years, 6 months ago

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2.8^n-5^=(8-5)k Bhaskar Immadisetty · 2 years, 6 months ago

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\(8^{n}-5^{n}=8^{n}-(8-3)^{n}=3A\) Rohit Sachdeva · 2 years, 6 months ago

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  1. Prove that sum of two integers within 100 is 101.
Uma Shankar · 2 years, 6 months ago

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