# Finding simplest radical form of a 4th root?

1. Jan 15, 2013

### Apollinaria

I haven't taken math in years and am having trouble understanding how to find simplest radical form of a 4√(x14).

I said x4√x10.

I realize I have 3 x4ths and x2 but I'm not sure if I can pull out more xs.

What are the rules for this? Ideas, insight?

2. Jan 15, 2013

### symbolipoint

For each occurance of x^4 as a factor in the term of the radicand, you have x to move to outside of the radical function.

$\sqrt[4]{x^{14}}$=$\sqrt[4]{x^4\cdot x^4\cdot x^4\cdot x^2}$

=$x^3\sqrt[4]{x^2}$

3. Jan 16, 2013

### sjb-2812

Can you simplify $\sqrt[4]{x^2}$ still further to $\sqrt{x}$, or does that fall foul of something like principal roots?

4. Jan 16, 2013

### I like Serena

Hi Apollinaria!

There are a few rules for dealing with radical form, powers from powers, and sums of powers.
Here's how it works in your case:
$$\sqrt[4]{x^{14}} = (x^{14})^{\frac 1 4} = x^{14 \cdot \frac 1 4} = x^{3 + \frac 1 2} = x^3 \cdot x^{\frac 1 2} = x^3 \sqrt x$$

5. Jan 16, 2013

### Khashishi

That doesn't work if x is negative. If you are considering all the complex roots, then $\sqrt[4]{x^2}$ has 4 roots and $\sqrt{x}$ has 2.

But, I think it works if you group together the solutions like $\sqrt[4]{x^2}$ = $\sqrt{x}$ or $\sqrt{-x}$.

6. Jan 16, 2013

### I like Serena

With the assumption that x is a non-negative real, it can be safely simplified.

If x can be a negative real, we have that √(x2) = |x| and 4√(x2)=√|x|.
However, in general we need to be very careful with negative real numbers and fractional powers.
They are generally not well-defined.
See for instance: http://en.wikipedia.org/wiki/Exponentiation#Rational_exponents
(The last couple of lines of the section.)

If x can be a complex number, it becomes even worse:
See for instance: http://en.wikipedia.org/wiki/Exponentiation#Failure_of_power_and_logarithm_identities