2.20: Adding, Subtracting, and Multiplying Radical Expressions

Last updated
Save as PDF

Page ID: 95224

$ \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } $ $ \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} $$\newcommand{\id}{\mathrm{id}}$ $ \newcommand{\Span}{\mathrm{span}}$ $ \newcommand{\kernel}{\mathrm{null}\,}$ $ \newcommand{\range}{\mathrm{range}\,}$ $ \newcommand{\RealPart}{\mathrm{Re}}$ $ \newcommand{\ImaginaryPart}{\mathrm{Im}}$ $ \newcommand{\Argument}{\mathrm{Arg}}$ $ \newcommand{\norm}[1]{\| #1 \|}$ $ \newcommand{\inner}[2]{\langle #1, #2 \rangle}$ $ \newcommand{\Span}{\mathrm{span}}$ $\newcommand{\id}{\mathrm{id}}$ $ \newcommand{\Span}{\mathrm{span}}$ $ \newcommand{\kernel}{\mathrm{null}\,}$ $ \newcommand{\range}{\mathrm{range}\,}$ $ \newcommand{\RealPart}{\mathrm{Re}}$ $ \newcommand{\ImaginaryPart}{\mathrm{Im}}$ $ \newcommand{\Argument}{\mathrm{Arg}}$ $ \newcommand{\norm}[1]{\| #1 \|}$ $ \newcommand{\inner}[2]{\langle #1, #2 \rangle}$ $ \newcommand{\Span}{\mathrm{span}}$$\newcommand{\AA}{\unicode[.8,0]{x212B}}$

Adding and Subtracting Radical Expressions

Adding radical expressions with the same index and the same radicand is just like adding like terms. We call radicals with the same index and the same radicand like radicals to remind us they work the same as like terms.

Like Radicals

Like radicals are radical expressions with the same index and the same radicand.

We add and subtract like radicals in the same way we add and subtract like terms. We know that $3x+8x$ is $11x$.Similarly we add $3 \sqrt{x}+8 \sqrt{x}$ and the result is $11 \sqrt{x}$.

Think about adding like terms with variables as you do the next few examples. When you have like radicals, you just add or subtract the coefficients. When the radicals are not like, you cannot combine the terms.

Example $\PageIndex{1}$

Simplify:

$2 \sqrt{2}-7 \sqrt{2}$
$5 \sqrt[3]{y}+4 \sqrt[3]{y}$
$7 \sqrt[4]{x}-2 \sqrt[4]{y}$

Solution:

$2 \sqrt{2}-7 \sqrt{2}$

Since the radicals are like, we subtract the coefficients.

$-5 \sqrt{2}$

$5 \sqrt[3]{y}+4 \sqrt[3]{y}$

Since the radicals are like, we add the coefficients.

$9 \sqrt[3]{y}$

$7 \sqrt[4]{x}-2 \sqrt[4]{y}$

The indices are the same but the radicals are different. These are not like radicals. Since the radicals are not like, we cannot subtract them.

You Try $\PageIndex{1}$

Simplify:

$5 \sqrt{3}-9 \sqrt{3}$
$5 \sqrt[3]{y}+3 \sqrt[3]{y}$
$5 \sqrt[4]{m}-2 \sqrt[3]{m}$

Answer

$-4 \sqrt{3}$
$8 \sqrt[3]{y}$
$5 \sqrt[4]{m}-2 \sqrt[3]{m}$

For radicals to be like, they must have the same index and radicand. When the radicands contain more than one variable, as long as all the variables and their exponents are identical, the radicands are the same.

Example $\PageIndex{2}$

Simplify:

$2 \sqrt{5 n}-6 \sqrt{5 n}+4 \sqrt{5 n}$
$\sqrt[4]{3 x y}+5 \sqrt[4]{3 x y}-4 \sqrt[4]{3 x y}$

Solution:

$2 \sqrt{5 n}-6 \sqrt{5 n}+4 \sqrt{5 n}$

Since the radicals are like, we combine them.

$0 \sqrt{5 n}$

Simplify.

$0$

$\sqrt[4]{3 x y}+5 \sqrt[4]{3 x y}-4 \sqrt[4]{3 x y}$

Since the radicals are like, we combine them.

$2 \sqrt[4]{3 x y}$

You Try $\PageIndex{2}$

Simplify:

$4 \sqrt{3 y}-7 \sqrt{3 y}+2 \sqrt{3 y}$
$6 \sqrt[3]{7 m n}+\sqrt[3]{7 m n}-4 \sqrt[3]{7 m n}$

Answer

$-\sqrt{3 y}$
$3 \sqrt[3]{7 m n}$

Remember that we always simplify radicals by removing the largest factor from the radicand that is a power of the index. Once each radical is simplified, we can then decide if they are like radicals.

Example $\PageIndex{3}$

Simplify:

$\sqrt{20}+3 \sqrt{5}$
$\sqrt[3]{24}-\sqrt[3]{375}$
$\frac{1}{2} \sqrt[4]{48}-\frac{2}{3} \sqrt[4]{243}$

Solution:

$\sqrt{20}+3 \sqrt{5}$

Simplify the radicals, when possible.

$\sqrt{4} \cdot \sqrt{5}+3 \sqrt{5}$

$2 \sqrt{5}+3 \sqrt{5}$

Combine the like radicals.

$5 \sqrt{5}$

$\sqrt[3]{24}-\sqrt[3]{375}$

Simplify the radicals.

$\sqrt[3]{8} \cdot \sqrt[3]{3}-\sqrt[3]{125} \cdot \sqrt[3]{3}$

$2 \sqrt[3]{3}-5 \sqrt[3]{3}$

Combine the like radicals.

$-3 \sqrt[3]{3}$

$\frac{1}{2} \sqrt[4]{48}-\frac{2}{3} \sqrt[4]{243}$

Simplify the radicals.

$\frac{1}{2} \sqrt[4]{16} \cdot \sqrt[4]{3}-\frac{2}{3} \sqrt[4]{81} \cdot \sqrt[4]{3}$

$\frac{1}{2} \cdot 2 \cdot \sqrt[4]{3}-\frac{2}{3} \cdot 3 \cdot \sqrt[4]{3}$

$\sqrt[4]{3}-2 \sqrt[4]{3}$

Combine the like radicals.

$-\sqrt[4]{3}$

You Try $\PageIndex{3}$

Simplify:

$\sqrt{27}+4 \sqrt{3}$
$4 \sqrt[3]{5}-7 \sqrt[3]{40}$
$\frac{1}{2} \sqrt[3]{128}-\frac{5}{3} \sqrt[3]{54}$

Answer

$7 \sqrt{3}$
$-10 \sqrt[3]{5}$
$-3 \sqrt[3]{2}$

In the next example, we will remove both constant and variable factors from the radicals. Now that we have practiced taking both the even and odd roots of variables, it is common practice at this point for us to assume all variables are greater than or equal to zero so that absolute values are not needed. We will use this assumption thoughout the rest of this chapter.

Example $\PageIndex{4}$

Simplify:

$9 \sqrt{50 m^{2}}-6 \sqrt{48 m^{2}}$
$\sqrt[3]{54 n^{5}}-\sqrt[3]{16 n^{5}}$

Solution:

$9 \sqrt{50 m^{2}}-6 \sqrt{48 m^{2}}$

Simplify the radicals.

$9 \sqrt{25 m^{2}} \cdot \sqrt{2}-6 \sqrt{16 m^{2}} \cdot \sqrt{3}$

$9 \cdot 5 m \cdot \sqrt{2}-6 \cdot 4 m \cdot \sqrt{3}$

$45 m \sqrt{2}-24 m \sqrt{3}$

The radicals are not like and so cannot be combined.

$\sqrt[3]{54 n^{5}}-\sqrt[3]{16 n^{5}}$

Simplify the radicals.

$\sqrt[3]{27 n^{3}} \cdot \sqrt[3]{2 n^{2}}-\sqrt[3]{8 n^{3}} \cdot \sqrt[3]{2 n^{2}}$

$3 n \sqrt[3]{2 n^{2}}-2 n \sqrt[3]{2 n^{2}}$

Combine the like radicals.

$n \sqrt[3]{2 n^{2}}$

You Try $\PageIndex{4}$

Simplify:

$\sqrt{27 p^{3}}-\sqrt{48 p^{3}}$
$\sqrt[3]{256 y^{5}}-\sqrt[3]{32 n^{5}}$

Answer

$-p \sqrt{3 p}$
$4 y \sqrt[3]{4 y^{2}}-2 n \sqrt[3]{4 n^{2}}$

Multiplying Radical Expressions

We have used the Product Property of Roots to simplify square roots by removing the perfect square factors. We can use the Product Property of Roots ‘in reverse’ to multiply square roots. Remember, we assume all variables are greater than or equal to zero.

We will rewrite the Product Property of Roots so we see both ways together.

Product Property of Roots

For any real numbers, $\sqrt[n]{a}$ and $\sqrt[n]{b}$, and for any integer $n≥2$

$\sqrt[n]{a b}=\sqrt[n]{a} \cdot \sqrt[n]{b} \quad \text { and } \quad \sqrt[n]{a} \cdot \sqrt[n]{b}=\sqrt[n]{a b}$

When we multiply two radicals they must have the same index. Once we multiply the radicals, we then look for factors that are a power of the index and simplify the radical whenever possible.

Multiplying radicals with coefficients is much like multiplying variables with coefficients. To multiply $4x⋅3y$ we multiply the coefficients together and then the variables. The result is $12xy$. Keep this in mind as you do these examples.

Example $\PageIndex{5}$

Simplify:

$(6 \sqrt{2})(3 \sqrt{10})$
$(-5 \sqrt[3]{4})(-4 \sqrt[3]{6})$

Solution:

$(6 \sqrt{2})(3 \sqrt{10})$

Multiply using the Product Property.

$18\sqrt{20}$

Simplify the radical.

$18 \sqrt{4} \cdot \sqrt{5}$

Simplify.

$18 \cdot 2 \cdot \sqrt{5}$

$36 \sqrt{5}$

$(-5 \sqrt[3]{4})(-4 \sqrt[3]{6})$

Multiply using the Product Property.

$20 \sqrt[3]{24}$

Simplify the radical.

$20 \sqrt[3]{8} \cdot \sqrt[3]{3}$

Simplify.

$20 \cdot 2 \cdot \sqrt[3]{3}$

$40 \sqrt[3]{3}$

You Try $\PageIndex{5}$

Simplify:

$(3 \sqrt{2})(2 \sqrt{30})$
$(2 \sqrt[3]{18})(-3 \sqrt[3]{6})$

Answer

$12 \sqrt{15}$
$-18 \sqrt[3]{2}$

We follow the same procedures when there are variables in the radicands.

Example $\PageIndex{6}$

Simplify:

$\left(10 \sqrt{6 p^{3}}\right)(4 \sqrt{3 p})$
$\left(2 \sqrt[4]{20 y^{2}}\right)\left(3 \sqrt[4]{28 y^{3}}\right)$

Solution:

$\left(10 \sqrt{6 p^{3}}\right)(4 \sqrt{3 p})$

Multiply.

$40 \sqrt{18 p^{4}}$

Simplify the radical.

$40 \sqrt{9 p^{4}} \cdot \sqrt{2}$

Simplify.

$40 \cdot 3 p^{2} \cdot \sqrt{3}$

$120 p^{2} \sqrt{3}$

b. When the radicands involve large numbers, it is often advantageous to factor them in order to find the perfect powers.

$\left(2 \sqrt[4]{20 y^{2}}\right)\left(3 \sqrt[4]{28 y^{3}}\right)$

Multiply.

$6 \sqrt[4]{4 \cdot 5 \cdot 4 \cdot 7 y^{5}}$

Simplify the radical.

$6 \sqrt[4]{16 y^{4}} \cdot \sqrt[4]{35 y}$

Simplify.

$6 \cdot 2 y \sqrt[4]{35 y}$

Multiply.

$12 y \sqrt[4]{35 y}$

You Try $\PageIndex{6}$

Simplify:

$\left(6 \sqrt{6 x^{2}}\right)\left(8 \sqrt{30 x^{4}}\right)$
$\left(-4 \sqrt[4]{12 y^{3}}\right)\left(-\sqrt[4]{8 y^{3}}\right)$

Answer

$36 x^{3} \sqrt{5}$
$8 y \sqrt[4]{3 y^{2}}$

Using Polynomial Multiplication to Multiply Radical Expressions

In the next a few examples, we will use the Distributive Property to multiply expressions with radicals. First we will distribute and then simplify the radicals when possible.

Example $\PageIndex{7}$

Simplify:

$\sqrt{6}(\sqrt{2}+\sqrt{18})$
$\sqrt[3]{9}(5-\sqrt[3]{18})$

Solution:

$\sqrt{6}(\sqrt{2}+\sqrt{18})$

Multiply.

$\sqrt{12}+\sqrt{108}$

Simplify.

$\sqrt{4} \cdot \sqrt{3}+\sqrt{36} \cdot \sqrt{3}$

Simplify.

$2 \sqrt{3}+6 \sqrt{3}$

Combine like radicals.

$8\sqrt{3}$

$\sqrt[3]{9}(5-\sqrt[3]{18})$

Distribute.

$5 \sqrt[3]{9}-\sqrt[3]{162}$

Simplify.

$5 \sqrt[3]{9}-\sqrt[3]{27} \cdot \sqrt[3]{6}$

Simplify.

$5 \sqrt[3]{9}-3 \sqrt[3]{6}$

You Try $\PageIndex{7}$

Simplify:

$\sqrt{8}(2-5 \sqrt{8})$
$\sqrt[3]{3}(-\sqrt[3]{9}-\sqrt[3]{6})$

Answer

$-40+4 \sqrt{2}$
$-3-\sqrt[3]{18}$

When we worked with polynomials, we multiplied binomials by binomials. Remember, this gave us four products before we combined any like terms. To be sure to get all four products, we organized our work—usually by the FOIL method.

Example $\PageIndex{8}$

Simplify:

$(3-2 \sqrt{7})(4-2 \sqrt{7})$
$(\sqrt[3]{x}-2)(\sqrt[3]{x}+4)$

Solution:

$(3-2 \sqrt{7})(4-2 \sqrt{7})$

Multiply.

$12-6 \sqrt{7}-8 \sqrt{7}+4 \cdot 7$

Simplify.

$12-6 \sqrt{7}-8 \sqrt{7}+28$

Combine like terms.

$40-14 \sqrt{7}$

$(\sqrt[3]{x}-2)(\sqrt[3]{x}+4)$

Multiply.

$\sqrt[3]{x^{2}}+4 \sqrt[3]{x}-2 \sqrt[3]{x}-8$

Combine like terms.

$\sqrt[3]{x^{2}}+2 \sqrt[3]{x}-8$

You Try $\PageIndex{8}$

Simplify:

$(2-3 \sqrt{11})(4-\sqrt{11})$
$(\sqrt[3]{x}+1)(\sqrt[3]{x}+3)$

Answer

$41-14 \sqrt{11}$
$\sqrt[3]{x^{2}}+4 \sqrt[3]{x}+3$

Example $\PageIndex{9}$

Simplify: $(3 \sqrt{2}-\sqrt{5})(\sqrt{2}+4 \sqrt{5})$

Solution:

$(3 \sqrt{2}-\sqrt{5})(\sqrt{2}+4 \sqrt{5})$

Multiply.

$3 \cdot 2+12 \sqrt{10}-\sqrt{10}-4 \cdot 5$

Simplify.

$6+12 \sqrt{10}-\sqrt{10}-20$

Combine like terms.

$-14+11 \sqrt{10}$

You Try $\PageIndex{9}$

Simplify: $(\sqrt{6}-3 \sqrt{8})(2 \sqrt{6}+\sqrt{8})$

Answer: $-12-20 \sqrt{3}$

Recognizing some special products made our work easier when we multiplied binomials earlier. This is true when we multiply radicals, too. The special product formulas we used are shown here.