Prove that sqrt(2) + sqrt(3) Cannot be a Rational Number

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Prove that sqrt(2) + sqrt(3) Cannot be a Rational Number

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Rational Roots theorem is a saviour. It makes life so much easier than showing it the manual way. Before I watched the video, I kinda expected that you would prove it by contradiction as one would normally do whenever a problem involves the word "CANNOT". Sometimes pure math is a matter of knowing as many theorems as possible and having the ability to coherently apply them.

albieadao
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Assume sqrt2 + sqrt3 is rational, then it can be written as p/q where p and q share no common factors. Then consider

(sqrt2 + sqrt3)^2 = p^2 / q^2
2 + 2sqrt(6) + 3 = p^2 / q^2
<=> sqrt(6) = (p^2/q^2 - 5)/2. This is still a rational number, so we can write it as a/b where a and b share no common factors.

However, this would imply that 6b^2 = a^2, which would imply that a^2 is divisible by 3, and consequently we can write a as 9k^2, and 2b^2 = 3k^2. However, this would by the same argument imply that b is also divisible by 3. This is a contradiction.

thecactus
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Please let me know if you have any questions.

vicheakeng
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i love this proof! Can't you also prove that sqrt2 is irrational by applying rational roots theorem to x^2-2=0 ??

nicholasroberts
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I use math for gambling just like I've beaten walkl st too. It doesn't matter what age u learn math but it has real applications out gere

johnbatchler
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Most of my math skills came out if wall st

johnbatchler
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U only need two yrs of math not four or higher

johnbatchler