Prove that $\sqrt{6}-\sqrt{2}-\sqrt{3}$ is irrational
My attempt:-
Suppose $\sqrt{6}-\sqrt{2}-\sqrt{3}$ is rational, then for some $x\in\mathbb{Q}$ we have $$\sqrt{6}-\sqrt{2}-\sqrt{3}=x$$
Rewriting this equation as $$\sqrt{6}-x=\sqrt{2}+\sqrt{3}$$
and now squaring this we get $$ 6-2x\sqrt{6}+x^2=5+2\sqrt{6}$$. This implies that $$\sqrt{6}=\frac{x^2-1}{2+2x}$$ but this is absurd as RHS of the above equation is rational but we know that $\sqrt6$ is irrational. Therefore , $\sqrt{6}-\sqrt{2}-\sqrt{3}$ is irrational. Does this look good? Have I written it properly? Is there any other proof besides this..like one using geometry? Thank you.
Best Answer
In your proof, after $6-2x\sqrt{6}+x^2=5+2\sqrt{6}$ we have that $$x^2+1=2(x+1)\sqrt{6}$$ If $x=-1$ then, from the above equation, it follows that $2=0$. Therefore $x$ is a rational number different from $-1$. After dividing by $2(x+1)\not=0$ we get $$\sqrt{6}=\frac{x^2+1}{2(x+1)}\in \mathbb{Q}.$$ Contradiction! Hence $x=\sqrt{6}-\sqrt{2}-\sqrt{3}$ is not a rational number.