## Multiply and Divide Radical Expressions

### Learning Outcomes

• Multiply and divide radical expressions
• Use the product raised to a power rule to multiply radical expressions
• Use the quotient raised to a power rule to divide radical expressions

You can do more than just simplify radical expressions. You can multiply and divide them, too. The product raised to a power rule that we discussed previously will help us find products of radical expressions. Recall the rule:

### A Product Raised to a Power Rule

For any numbers a and b and any integer x: ${{(ab)}^{x}}={{a}^{x}}\cdot {{b}^{x}}$

For any numbers a and b and any positive integer x: ${{(ab)}^{\frac{1}{x}}}={{a}^{\frac{1}{x}}}\cdot {{b}^{\frac{1}{x}}}$

For any numbers a and b and any positive integer x: $\sqrt[x]{ab}=\sqrt[x]{a}\cdot \sqrt[x]{b}$

The Product Raised to a Power Rule is important because you can use it to multiply radical expressions. Note that you cannot multiply a square root and a cube root using this rule. The indices of the radicals must match in order to multiply them. In our first example, we will work with integers, and then we will move on to expressions with variable radicands.

### Example

Simplify. $\sqrt{18}\cdot \sqrt{16}$

You may have also noticed that both $\sqrt{18}$ and $\sqrt{16}$ can be written as products involving perfect square factors. How would the expression change if you simplified each radical first, before multiplying? In the next example, we will use the same product from above to show that you can simplify before multiplying and get the same result.

### Example

Simplify. $\sqrt{18}\cdot \sqrt{16}$

In both cases, you arrive at the same product, $12\sqrt{2}$. It does not matter whether you multiply the radicands or simplify each radical first.

You multiply radical expressions that contain variables in the same manner. As long as the roots of the radical expressions are the same, you can use the Product Raised to a Power Rule to multiply and simplify. Look at the two examples that follow. In both problems, the Product Raised to a Power Rule is used right away and then the expression is simplified. Note that we specify that the variable is non-negative, $x\ge 0$, thus allowing us to avoid the need for absolute value.

### Example

Simplify. $\sqrt{12{{x}^{4}}}\cdot \sqrt{3x^2}$, $x\ge 0$

### Analysis of the Solution

Even though our answer contained a variable with an odd exponent that was simplified from an even indexed root, we don’t need to write our answer with absolute value because we specified before we simplified that $x\ge 0$. It is important to read the problem very well when you are doing math.  Even the smallest statement like $x\ge 0$ can influence the way you write your answer.

In our next example, we will multiply two cube roots.

### Example

Simplify. $\sqrt[3]{{{x}^{5}}{{y}^{2}}}\cdot 5\sqrt[3]{8{{x}^{2}}{{y}^{4}}}$

In the following video, we present more examples of how to multiply radical expressions.

This next example is slightly more complicated because there are more than two radicals being multiplied. In this case, notice how the radicals are simplified before multiplication takes place. Remember that the order you choose to use is up to you—you will find that sometimes it is easier to multiply before simplifying, and other times it is easier to simplify before multiplying. With some practice, you may be able to tell which is easier before you approach the problem, but either order will work for all problems.

### Example

Simplify. $2\sqrt[4]{16{{x}^{9}}}\cdot \sqrt[4]{{{y}^{3}}}\cdot \sqrt[4]{81{{x}^{3}}y}$, $x\ge 0$, $y\ge 0$

In the following video, we show more examples of multiplying cube roots.

You can use the same ideas to help you figure out how to simplify and divide radical expressions. Recall that the Product Raised to a Power Rule states that $\sqrt[x]{ab}=\sqrt[x]{a}\cdot \sqrt[x]{b}$. Well, what if you are dealing with a quotient instead of a product?

There is a rule for that, too. The Quotient Raised to a Power Rule states that ${{\left( \frac{a}{b} \right)}^{x}}=\frac{{{a}^{x}}}{{{b}^{x}}}$. Again, if you imagine that the exponent is a rational number, then you can make this rule applicable for roots as well: ${{\left( \frac{a}{b} \right)}^{\frac{1}{x}}}=\frac{{{a}^{\frac{1}{x}}}}{{{b}^{\frac{1}{x}}}}$, so $\sqrt[x]{\frac{a}{b}}=\frac{\sqrt[x]{a}}{\sqrt[x]{b}}$.

### A Quotient Raised to a Power Rule

For any real numbers a and b (b ≠ 0) and any positive integer x: ${{\left( \frac{a}{b} \right)}^{\frac{1}{x}}}=\frac{{{a}^{\frac{1}{x}}}}{{{b}^{\frac{1}{x}}}}$

For any real numbers a and b (b ≠ 0) and any positive integer x: $\sqrt[x]{\frac{a}{b}}=\frac{\sqrt[x]{a}}{\sqrt[x]{b}}$

As you did with multiplication, you will start with some examples featuring integers before moving on to more complex expressions like $\frac{\sqrt[3]{24x{{y}^{4}}}}{\sqrt[3]{8y}}$.

### Example

Simplify. $\sqrt{\frac{48}{25}}$

### Example

Simplify. $\sqrt[3]{\frac{640}{40}}$

That was a lot of effort, but you were able to simplify using the Quotient Raised to a Power Rule. What if you found the quotient of this expression by dividing within the radical first and then took the cube root of the quotient?

Now take another look at that problem using this approach.

### Example

Simplify. $\frac{\sqrt[3]{640}}{\sqrt[3]{40}}$

Notice how much more straightforward the approach was.

As with multiplication, the main idea here is that sometimes it makes sense to divide and then simplify, and other times it makes sense to simplify and then divide. Whichever order you choose, though, you should arrive at the same final expression.

Now let us turn to some radical expressions containing division. Notice that the process for dividing these is the same as it is for dividing integers.

### Example

Simplify. $\frac{\sqrt{30x}}{\sqrt{10x}},x>0$

### Example

Simplify. $\frac{\sqrt[3]{24x{{y}^{4}}}}{\sqrt[3]{8y}},\,\,y\ne 0$

As you become more familiar with dividing and simplifying radical expressions, make sure you continue to pay attention to the roots of the radicals that you are dividing. For example, while you can think of $\frac{\sqrt{8{{y}^{2}}}}{\sqrt{225{{y}^{4}}}}$ as being equivalent to $\sqrt{\frac{8{{y}^{2}}}{225{{y}^{4}}}}$ since both the numerator and the denominator are square roots, notice that you cannot express $\frac{\sqrt{8{{y}^{2}}}}{\sqrt[4]{225{{y}^{4}}}}$ as $\sqrt[4]{\frac{8{{y}^{2}}}{225{{y}^{4}}}}$. In this second case, the numerator is a square root and the denominator is a fourth root.
The Product Raised to a Power Rule and the Quotient Raised to a Power Rule can be used to simplify radical expressions as long as the roots of the radicals are the same. The Product Rule states that the product of two or more numbers raised to a power is equal to the product of each number raised to the same power. The same is true of roots: $\sqrt[x]{ab}=\sqrt[x]{a}\cdot \sqrt[x]{b}$. When dividing radical expressions, the rules governing quotients are similar: $\sqrt[x]{\frac{a}{b}}=\frac{\sqrt[x]{a}}{\sqrt[x]{b}}$.