# Math Help - proof help

1. ## proof help

Prove that the square root of 6 is irrational.

Sorry i don't know how to do the square root in here.

I would use the proof by contradiction method for this.

So let's assume that ${\sqrt {6}}$ is rational.

By definition, that means there are two integers a and b with no common divisors where:

${\frac{a}{b}} = {\sqrt {6}}$

So let's multiply both sides by themselves:

$({\frac{a}{b}})({\frac{a}{b}}) = ({\sqrt {6}})({\sqrt {6}})$

${\frac{a{^2}}{b{^2}}} = 6$

$a{^2} = 6b{^2}$

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 $a{^2}$ must be even. But any odd number times itself is odd, so if $a{^2}$ 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:

$a{^2} = 6b{^2}$

$(2c){^2} = (2)(3)b{^2}$

$2c{^2} = 3b{^2}$

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 ${\sqrt {6}}$ cannot be rational.