Why is the square root of 24 irrational?

Answer 1

If it were rational, then the square root of 6 would be also since sqrt 24 is sqrt(4x6)=2xsqrt(6) and here is a proof the sqrt(6) is irrational So let's assume by contradiction that the square root of 6 is rational. By definition, that means there are two integers a and b with no common divisors where: a/b = square root of 6. So let's multiply both sides by themselves:

(a/b)(a/b) = (square root of 6)(square root of 6)

a2/b2 = 6

a2 = 6b2

But this last statement means the RHS (right hand side) is even, because it is a product of integers and one of those integers (at least) is even. So a2 must be even. But any odd number times itself is odd, so if a2 is even, then a is even. Since a is even, there is some integer c that is half of a, or in other words:

2c = a.

Now let's replace a with 2c:

a2 = 6b2

(2c)2 = (2)(3)b2

2c2 = 3b2

But now we can argue the same thing for b, because the LHS is even, so the RHS must be even and that means b is even. Now this is the contradiction: if a is even and b is even, then they have a common divisor (2). Then our initial assumption must be false, so the square root of 6 cannot be rational.