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Uploaded At Apr 11, 2024 ^^
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RYD date created : 2024-11-09T20:23:08.064566Z
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Top Comments of this video!! :3
Nice proof, I've haven't seen a visual one of this before! If someone's still not getting it:
If √2 were rational, there would have to exist (finite) integers a and b so that a/b = √2.
a/b = √2, square both sides
a²/b² = 2, multiply by b²
a² = 2b²
So proving the irrationality of √2 is equivalent to proving there is no square of an integer, that is double the square of another integer. The proof by contradiction in the video assumes the opposite. There are no finite a and b to satisfy these equations, so √2 has to be irrational.
Correct me if I'm wrong, thanks!
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I prefer the following instead of infinite descent:
Take the smallest counterexample. By the step, there's a smaller one. This is a contradiction.
You can note that this is essentially a special case of a reasoning that under a slightly different formulation is known as transfinite induction. It's similar to induction, but more powerful. It's also proven differently. Normal induction is proven from the definition of natural numbers (and 0) as ones reachable by the operations. (By “reachable”, I mean present in every set closed under them.) Transfinite induction is essentially the well-orderedness (existence of the smallest in every subset) of the natural numbers (or whatever set you're using), although for N it may be easier formulated as the normal induction over prefixes
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simplify sqrt to r
a/b = r(2) for some a,b inZ+, gcd(a,b)=1
a=r(2)b
aa=2bb
lhs has factor 2 but there are two a so 4 | a
Thus 4 | 2bb and this imply 2 | bb
Which means 2 is factor of b
So gcd(a,b) does not equal to one. Therefore, no such coprime integer pair (a,b) exists
I think this is the same to the video but in different way of recursively decreasing a and b
If we use the well ordered property of integer and state (a,b) is the least element satisfying the equation then we do not have to state the recursive part
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Nice proof, but I’ll propose my personal favorite:
The exponents in the prime factorization of a perfect square are always even. When you multiply this perfect square by a prime number, the exponent in that prime factor becomes odd. This means that you cannot multiply a perfect square by any prime number to get a perfect square. Therefore, the square root of any prime number is irrational.
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@YouTube_username_not_found
7 months ago
Infinite descent!
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