# How to solve $x^{1/2}-x^{1/3} = 0$

How can I solve the following equation? I really can't figure out how to solve it:

$x^{1/2}-x^{1/3} = 0$

Thank you.

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2
Please, try to make the title of your question more informative. E.g., Why does $a<b$ imply $a+c<b+c$? is much more useful for other users than A question about inequality. From How can I ask a good question?: Make your title as descriptive as possible. In many cases one can actually phrase the title as the question, at least in such a way so as to be comprehensible to an expert reader. – Julian Kuelshammer Dec 13 '12 at 15:34

## 7 Answers

$$\begin{eqnarray*}x^{1/2}-x^{1/3} &=& 0 \\ \\ \iff x^{3/6} - x^{2/6} &=& 0 \\ \\ \iff x^{2/6}(x^{1/6} - 1)&=& 0 \\ \\ \iff [x^{1/6} = 1\text{ or}\;x^{2/6} &=& 0] \\ \\ \iff [x = 1\text{ or}\;x &=& 0]\end{eqnarray*}$$

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Another thumbs-up from me! +1 – Amzoti yesterday

Putting $x=y^6, y^3=y^2,y^2(y-1)=0\implies y=0$ or $1\implies x=0$ or $1$

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6
Observe that $6$ is chosen as lcm$(2,3)=6$ – lab bhattacharjee Dec 13 '12 at 15:16

You might have heard this a thousand times, but the equation really does speak here!

What it says is that when you raise a number ($x$) to two different powers ($\frac 12$ and $\frac 13$) they turn out to be equal (their difference is $0$).

The only two numbers that come to mind are $0$ and $1$. $\Big[ \forall n \in \mathbb R, \;\; 0^n=0$ and $1^n=1 \Big]$

So the solution should be $x=0\ \text{ or } \ 1$

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6
+1 for the very nice first two paragraphs… but I almost took it back for the disclaimer! If an intuitive approach isn’t mathematically sound, the response shouldn’t be “ah well, accept that it’s not so good, and use an alternative solution”. The response should be “OK, so which bit isn’t mathematically sound, and how can we make it so?” With a little work in the third paragraph, your answer can become a solution that’s both mathematically sound and more intuitively clear than some of the other answers here. – Peter LeFanu Lumsdaine Dec 14 '12 at 3:35
I think that this can be trivially made "mathematically sound" and it's a nice way to show how carefully reading and understanding what the problem asks one to show is important before one starts writing. – Nik Bougalis Dec 14 '12 at 4:50

(Henceforth, $x$ is a variable denoting a non-negative real number)

$$x^{1/2} - x^{1/3} = 0$$ if and only if $$x^{1/2} = x^{1/3}$$ if and only if $$\frac{x^{1/2}}{x^{1/3}} = 1 \qquad \text{or} \qquad x^{1/3} = 0$$ if and only if $$x^{\frac{1}{2} - \frac{1}{3}} = 1 \qquad \text{or} \qquad x = 0$$ if and only if $$x^{1/6} = 1 \qquad \text{or} \qquad x = 0$$ if and only if $$x = 1 \qquad \text{or} \qquad x = 0$$

$$x^{1/2} - x^{1/3} = 0$$ if and only if $$x^{1/2} = x^{1/3}$$ if and only if $$x = x^{2/3}$$ if and only if $$x^3 = x^2$$ if and only if $$x = 1 \qquad \text{or} \qquad x^2 = 0$$ if and only if $$x = 1 \qquad \text{or} \qquad x = 0$$

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$x^{1/2}-x^{1/3} = 0$

$x^{1/2}=x^{1/3}$

$(x^{1/2})^6=(x^{1/3})^6$

$x^3=x^2$

$x^3-x^2=0$

$x^2(x-1)=0$

$x^2 = 0$ or $x-1=0$

$x = 0$ or $x = 1$

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Similar to the first answer.

$$x^{\frac{1}{2}}-x^{\frac{1}{3}}=0$$ Rewrite $x^{\frac{1}{2}}$ as something clever: $$x^{\frac{3}{6}}-x^{\frac{1}{3}}=0$$ (In case that's illegible, that's x^(3/6))

Now factor out $x^{\frac{1}{3}}$: $$x^{\frac{1}{3}}(x^{\frac{2}{6}}-1)=0$$

Now divide both sides by $x^{\frac{1}{3}}$. Note that this assumes $x \neq 0$; we will have to go back and check for that case later.

$$x^{\frac{2}{6}}-1=0$$ $$x^{\frac{2}{6}}=1$$ $$x^{\frac{1}{3}}=1$$ $$(x^{\frac{1}{3}})^3=1^3$$ $$x=1$$

Now let's go back and check if it works for $x=0$:

$$(0)^{\frac{1}{2}}-(0)^{\frac{1}{3}}=0$$

Yep, that equation holds!

So either $x=1$ or $x=0$.

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This is late, but for problems like these, I use Wolfram Alpha if I'm stuck. If you log in, it lets you see 3 step-by-step solutions daily.

Here's the page for this problem.

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