Learning Outcomes
- Add and subtract radical expressions.
- Find products of two or more expressions which include radical terms.
- Multiply or divide radical expressions with different indices including expressions written in function notation.
Similar to the previous two sections, we now ask: Can we add radicals, and if so, how? Consider:
[latex]\require{color}\sqrt{4}+\sqrt{9}[/latex]
If we add the radicands, we get [latex]\color{Red}{\sqrt{4+9}=\sqrt{13}}.[/latex] However, this is incorrect! For this problem we could have instead just evaluated each radical separately and obtained [latex]\color{Green}{\sqrt{4}+\sqrt{9}=2+3=5},[/latex] which is the correct answer. This means adding the radicands is NOT correct. Let’s state this here as an important fact:
In general, [latex]\sqrt[n]{a} + \sqrt[n]{b} \neq \sqrt[n]{a + b}.[/latex]
Our exponential properties are not helpful either, because we don’t have any properties dealing with multiple terms added or subtracted. In particular, [latex](a+b)^n \neq a^n+b^n[/latex] as a general rule.
To add and subtract radicals, we need to borrow the idea of like terms. Consider [latex]\sqrt{2} + \sqrt{2}.[/latex] We can say that we have 2 copies of [latex]\sqrt{2},[/latex] or symbolically we have [latex]2\sqrt{2}.[/latex]
One helpful tip is to think of radical expressions like polynomials. In a polynomial, two terms are like terms if the variables and their exponents are the same, for example [latex]3xy^2[/latex] and [latex]8xy^2[/latex]. Similarly, two radical expressions are like terms if their indices and radicands are equal, for example [latex]3\sqrt[5]{3x}[/latex] and [latex]-5\sqrt[5]{3x}.[/latex]
LIKE TERMS IN RADICAL EXPRESSIONS
In an expression with radicals, two radical terms are like terms if they have the same radicand and the same index.
Here are some full examples of addition and subtraction.
Example
Simplify each expression by combining like terms.
- [latex] 3\sqrt{11}+7\sqrt{11}[/latex]
- [latex] 5\sqrt{2}+\sqrt{3}+4\sqrt{3}+2\sqrt{2}[/latex]
- [latex] 5\sqrt{13}-3\sqrt{13}[/latex]
- [latex] 3\sqrt{x}+12\sqrt[3]{x}-7\sqrt{x}[/latex]
In the following videos, we show more examples of how to identify and combine like radical terms.
Sometimes it may not be clear immediately that two radical terms can be combined. Consider the following expression.
[latex]5\sqrt{2}+2\sqrt{18}[/latex]
They have different radicands, so they cannot be added as written. However, the second radical can be simplified:
[latex] \begin{align} &\quad 5\sqrt{2}+2\sqrt{18}\\=&\quad 5\sqrt{2}+2\sqrt{9}\sqrt{2}\\=&\quad 5\sqrt{2}+2\cdot 3\sqrt{2}\\=&\quad 5\sqrt{2}+6\sqrt{2}\\=&\quad 11\sqrt{2}\end{align}[/latex]
After simplifying the second radical, the expression had two like terms and we could combine them. Always simplify all radicals in the expression first before attempting to find like terms. Here are some more examples.
Example
Simplify each expression by combining like terms if possible. Assume all variables represent nonnegative quantities.
- [latex] \sqrt{28}-3\sqrt{63}[/latex]
- [latex] 2\sqrt[3]{40}+\sqrt[3]{135}[/latex]
- [latex] x\sqrt[3]{x{{y}^{4}}}+y\sqrt[3]{{{x}^{4}}y}[/latex]
- [latex] 5\sqrt[4]{{{a}^{5}}b}-a\sqrt[4]{16ab}[/latex]
The following videos show more examples of combining radical expressions that require simplification first.
Products of Radical Expressions with Multiple Terms
Just like with addition and subtraction, we can use tools from polynomials to work with radical expressions of multiple terms. So, although the expression [latex] \sqrt{x}(3\sqrt{x}-5)[/latex] may look different from [latex] a(3a-5)[/latex], you can treat them the same way. The second expression can be resolved by distributing, [latex] 3a^2-5a[/latex]. Let’s see how to distribute the radical expression.
Example
Multiply [latex] \sqrt{x}(3\sqrt{x}-5).[/latex] Assume all variables represent nonnegative quantities.
In these next two examples, each term contains a radical.
Example
Simplify. [latex] 7\sqrt{x}\left( 2\sqrt{xy}+\sqrt{y} \right)[/latex] Assume all variables represent nonnegative quantities.
Example
Simplify. [latex] \sqrt[3]{a}\left( 2\sqrt[3]{{{a}^{2}}}-4\sqrt[3]{{{a}^{5}}}+8\sqrt[3]{{{a}^{8}}} \right)[/latex]
In the following video, we show more examples of how to multiply radical expressions using the Distributive Property.
After you have worked with radical expressions a bit more, you may feel more comfortable identifying quantities such as [latex] \sqrt{x}\cdot \sqrt{x}=x[/latex] without going through the intermediate step of finding that [latex] \sqrt{x}\cdot \sqrt{x}=\sqrt{{{x}^{2}}}[/latex].
Multiply Binomial Expressions that Contain Radicals
You can use the same technique for multiplying binomials to multiply binomial expressions with radicals. To multiply [latex] \left( 2x+5 \right)\left( 3x-2 \right),[/latex] we would typically use the FOIL shortcut. Let’s see how that works if we replace each [latex]x[/latex] with [latex]\sqrt{6}[/latex].
Example
Multiply and simplify. [latex] \left( 2\sqrt{6}+5 \right)\left( 3\sqrt{6}-2 \right)[/latex]
Here are some additional examples which will be extremely useful later.
Example
Multiply and simplify. Assume all variables represent nonnegative quantities.
- [latex]\left(4\sqrt{2}-3\sqrt{7}\right)\left(3\sqrt{2}+\sqrt{7}\right)[/latex]
- [latex]\left(5-2\sqrt{x}\right)^2[/latex]
- [latex]\left(4+\sqrt{11}\right)\left(4-\sqrt{11}\right)[/latex]
In the last part of the previous example, our final answer contained no radicals. The binomials we multiplied are called conjugates. In the next section we will learn more about conjugates and why it is useful to multiply radical expressions and get whole numbers.
In the following video, we show more examples of how to multiply two binomials that contain radicals.
Multiplying and Dividing with Different Indices
It is possible to multiply and divide radical expressions that have different indices, but we cannot use the same rules we did before. Let’s see two examples of how this is possible.
Example 1: Same Radicand
[latex] \sqrt[3]{5} \cdot \sqrt[4]{5} [/latex]
We use a different exponent property to simplify this: the Product Rule of Exponents, which says [latex]a^m\cdot a^n=a^{m+n}.[/latex]
[latex]\begin{align} &\sqrt[3]{5} \cdot \sqrt[4]{5}\\ =\quad &5^{\frac{1}{3}}\cdot 5^{\frac{1}{4}}\\ =\quad &5^{\frac{1}{3}+\frac{1}{4}}\\ =\quad &5^{\frac{4}{12}+\frac{3}{12}}\\ =\quad &5^{\frac{7}{12}}=\sqrt[12]{5^7}\end{align}[/latex]
This process only works if the radicand is the same in both expressions. We see an important theme which will be used in Example 2 though – often combining radicals turns into a problem of finding a common denominator (which becomes a common index).
Example 2: Different Radicands
[latex]\begin{align}&\sqrt[3]{5}\cdot\sqrt{2}\\ =\quad&5^{\frac{1}{3}}\cdot2^{\frac{1}{2}}\\ =\quad&5^{\frac{2}{6}}\cdot2^{\frac{3}{6}}&&\color{blue}{\textsf{Find common denominator}}\\ =\quad&\left(5^2\cdot2^3\right)^{\frac{1}{6}}&&\color{blue}{\textsf{Power of a Product Rule}}\\ =\quad&\sqrt[6]{200} \end{align}[/latex]
The key here is that the common denominator became the new index on the combined radical. Here are a few more examples to try on your own.
Example
Perform the indicated operation and simplify. Write answers in radical form. Assume all variables represent positive quantities.
- [latex] \sqrt[4]{2}\cdot\sqrt[5]{4}[/latex]
- [latex] \sqrt[7]{ab^4}\cdot\sqrt[6]{a^5b}[/latex]
- Given [latex]f(x)=\sqrt[3]{x^2}[/latex] and [latex]g(x)=\sqrt[6]{x}[/latex], find and simplify [latex]\left(\dfrac{f}{g}\right)(x).[/latex]
Summary
Adding or subtracting radical terms is possible when the index and the radicand of two or more radicals are the same. Radicals with the same index and radicand are known as like radicals. It is often helpful to treat radicals just as you would treat variables; like radical terms can be added and subtracted in the same way that like variable terms can be added and subtracted. Sometimes, you will need to simplify a radical expression before it is possible to add or subtract like terms.
To multiply radical expressions that contain more than one term, use the same method that you use to multiply polynomials. First, use the Distributive Property (or the shortcut FOIL method) to multiply the terms. Then, apply the rule [latex] \sqrt[n]{a}\cdot \sqrt[n]{b}=\sqrt[n]{ab}[/latex] wherever necessary to multiply and simplify. Finally, combine like terms.
Radicals with different indices can be multiplied or divided, usually by converting the radicals to exponent notation first.