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2.06 Fractional indices

Lesson

We want to look at expressions of the form $x^{\frac{m}{n}}$xmn. Let's make a connection to previous knowledge and start with a particular example of $x^{\frac{1}{2}}$x12.

Consider the process below using the index laws we have previously looked at:

$x^{\frac{2}{2}}$x22 $=$= $x$x
$\left(x^{\frac{1}{2}}\right)^2$(x12)2 $=$= $x$x
$\left(x^{\frac{1}{2}}\right)^2$(x12)2 $=$= $\left(\sqrt{x}\right)^2$(x)2
$x^{\frac{1}{2}}$x12 $=$= $\sqrt{x}$x

 

Fractional indices

$x^{\frac{1}{n}}=\sqrt[n]{x}$x1n=nx

 

More fractional indices

You may also see questions with more complicated fractional indices, such as $x^{\frac{3}{2}}$x32. We could express this as a power of a power, using the rule $\left(x^a\right)^b=x^{ab}$(xa)b=xab, as follows:

$x^{\frac{3}{2}}=\left(x^3\right)^{\frac{1}{2}}$x32=(x3)12$=$=$\sqrt{x^3}$x3.

As such, the numerator in the fractional index can be expressed as a power and the denominator in the fractional index can be expressed as a root.

More generally, this rule states:

Fractional indices

$x^{\frac{m}{n}}=\sqrt[n]{x^m}$xmn=nxm

$x^{\frac{m}{n}}=\left(\sqrt[n]{x}\right)^m$xmn=(nx)m

 

When solving problems with fractional indices, it doesn't matter whether you start with the powers or the roots (although you might find it easier to do it one way than the other).

For example, let's look at $16^{\frac{3}{2}}$1632:

Starting with the root:

$16^{\frac{3}{2}}$1632 $=$= $\left(\sqrt{16}\right)^3$(16)3
  $=$= $4^3$43
  $=$= $64$64

Now let's start with the powers:

$16^{\frac{3}{2}}$1632 $=$= $\sqrt{16^3}$163
  $=$= $\sqrt{4096}$4096
  $=$= $64$64

You can see that we get the same answer both ways, but the second approach led to working with some much larger numbers

Practice questions

Question 1

Consider the following.

  1. Rewrite $x^{\frac{1}{3}}$x13 in surd form.

  2. Evaluate $\sqrt[3]{x}$3x for when $x=8$x=8.

Question 2

Write the following with a fractional index:

$\sqrt[8]{23}$823

Question 3

Simplify the following, leaving your answer in index form:

$5^{-\frac{2}{9}}\times5^{\frac{4}{9}}$529×549.

QUESTION 4

Write the following as a simplified fraction:

$\left(\frac{16}{9}\right)^{-\frac{1}{2}}$(169)12.

Outcomes

VCMNA355 (10a)

Define rational and irrational numbers and perform operations with surds and fractional indices.

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