Rational Exponents and Radicals
Emma is laying a square patio with an area of $50$ ft². Her calculator says each side is $7.0710678\ldots$ ft. That number is rounded, and it never ends. What is the exact length of the side, and how can you work with it without a calculator? The answer is to see a root for what it really is: a power whose exponent is a fraction.
Roots as powers
Why would a root be a power? Take the exponent rule $a^m \cdot a^n = a^{m+n}$ and apply it to $a^{1/2}$:
$$a^{1/2} \cdot a^{1/2} = a^{1/2 + 1/2} = a^1 = a$$A number that multiplies by itself to give $a$ is exactly $\sqrt{a}$. So $a^{1/2}$ has to be $\sqrt{a}$ if the exponent rules are going to keep working. The same idea works for cube roots, fourth roots, and so on.
On this page, every variable under an even root represents a positive number. (Otherwise $\sqrt{x^2} = |x|$, not just $x$.)
Express the following powers with fractional exponents as a radical and calculate their exact value if it is an integer: a) $27^{\frac{1}{3}}$ b) $16^{\frac{3}{4}}$ c)…
Express the following powers with fractional exponents as a radical and calculate their exact value if it is an integer:
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a) $27^{\frac{1}{3}}$
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b) $16^{\frac{3}{4}}$
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c) $5^{\frac{2}{3}}$
Write the following radicals in the form of a power with a fractional exponent and simplify the fraction of the exponent: a) $\sqrt[6]{x^2}$ b) $\sqrt[15]{a^5}$ c)…
Write the following radicals in the form of a power with a fractional exponent and simplify the fraction of the exponent:
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a) $\sqrt[6]{x^2}$
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b) $\sqrt[15]{a^5}$
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c) $\sqrt[8]{2^6}$
Simplifying radicals
A radical is in simplest form when no factor of the radicand is a perfect $n$th power. Some problems call pulling a factor out of a radical extracting factors, and the reverse introducing them. Both are just exponent rules in disguise.
To pull factors out, look for the largest perfect power that divides the radicand. It helps to know these:
| Index | Perfect powers |
|---|---|
| 2 (squares) | 4, 9, 16, 25, 36, 49, 64, 81, 100 |
| 3 (cubes) | 8, 27, 64, 125 |
| 4 (fourth powers) | 16, 81, 256 |
A square with area $A$ has side $\sqrt{A}$, and its perimeter is $4\sqrt{A}$.
Back to the patio: the side is $\sqrt{50} = \sqrt{25\cdot 2} = 5\sqrt{2}$ ft, about $7.07$ ft, and the border around it is $4 \cdot 5\sqrt{2} = 20\sqrt{2}$ ft.
Extract all possible factors from inside the following radicals: a) $\sqrt{50}$ b) $\sqrt[3]{16}$ c) $\sqrt[4]{x^7 y^5}$
Extract all possible factors from inside the following radicals:
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a) $\sqrt{50}$
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b) $\sqrt[3]{16}$
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c) $\sqrt[4]{x^7 y^5}$
Introduce the factors inside the radical and simplify the resulting expression if possible: a) $3 \sqrt{2}$ b) $x^2 \sqrt[3]{x}$ c) $2a \sqrt[4]{3a^2}$, with $a \ge 0$
Introduce the factors inside the radical and simplify the resulting expression if possible:
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a) $3 \sqrt{2}$
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b) $x^2 \sqrt[3]{x}$
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c) $2a \sqrt[4]{3a^2}$, with $a \ge 0$
An agricultural cooperative wants to fence a square-shaped plot of land whose total area is $1250 \text{ ft}^2$. Calculate the exact length of the side of the land by…
An agricultural cooperative wants to fence a square-shaped plot of land whose total area is $1250 \text{ ft}^2$. Calculate the exact length of the side of the land by extracting factors from the radical and determine how many feet of fence they will need in total, leaving the result in its simplest radical form.
Adding and subtracting radicals
Radicals work like variables when you add them. $4x + 2x = 6x$, and in the same way
$$4\sqrt{3} + 2\sqrt{3} = 6\sqrt{3}$$These are like radicals: same index and same radicand. You add the coefficients and keep the radical. But $\sqrt{2} + \sqrt{3}$ cannot be combined, just as $x + y$ cannot.
Radicals that look different may become like radicals after you simplify them. So simplify first.
A landscape architect is designing a green area in the shape of a scalene triangle and needs to calculate its perimeter to order a fence. The plans indicate that the…
A landscape architect is designing a green area in the shape of a scalene triangle and needs to calculate its perimeter to order a fence. The plans indicate that the sides measure $\sqrt{48}$ ft, $\sqrt{75}$ ft and $\sqrt{12}$ ft. Simplify the radicals by extracting factors and calculate the exact perimeter expressed as a single radical.
In maritime navigation, the distance $d$ (in miles) to the visible horizon for an observer at a height $h$ (in feet) above sea level is approximated by the formula…
In maritime navigation, the distance $d$ (in miles) to the visible horizon for an observer at a height $h$ (in feet) above sea level is approximated by the formula $d = \sqrt{13h}$. Two observers are at different points on a cliff at heights of $h_1 = 52$ ft and $h_2 = 117$ ft respectively. Calculate the exact difference in their distances to the visible horizon, simplifying the expression $\sqrt{13 \cdot 117} - \sqrt{13 \cdot 52}$ by extracting factors.
Multiplying, dividing and raising radicals (same index)
When two radicals have the same index, you can combine them under one root. To raise a radical to a power, put the power inside.
Simplest form also means no radical in the denominator. Clearing it like this is called rationalizing the denominator.
Perform the following products of radicals with the same index and extract factors from the result if possible: a) $\sqrt{3} \cdot \sqrt{12}$ b)…
Perform the following products of radicals with the same index and extract factors from the result if possible:
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a) $\sqrt{3} \cdot \sqrt{12}$
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b) $\sqrt[3]{4} \cdot \sqrt[3]{10}$
Calculate and simplify the result of the following divisions of radicals: a) $\frac{\sqrt{72}}{\sqrt{2}}$ b) $\frac{\sqrt[3]{40}}{\sqrt[3]{5}}$
Calculate and simplify the result of the following divisions of radicals:
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a) $\frac{\sqrt{72}}{\sqrt{2}}$
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b) $\frac{\sqrt[3]{40}}{\sqrt[3]{5}}$
Calculate the following powers of radicals, introducing the exponent into the radicand and expressing the result in the simplest possible form: a) $(\sqrt[3]{5})^6$ b)…
Calculate the following powers of radicals, introducing the exponent into the radicand and expressing the result in the simplest possible form:
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a) $(\sqrt[3]{5})^6$
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b) $(\sqrt{x^3})^4$
Radicals with different indices
If the indices are different, you cannot just merge the radicands. First rewrite both radicals as powers and give the exponents a common denominator. That denominator is the common index, the LCM of the indices.
For the manufacturing of three electronic components, protective sheets are required whose thicknesses, measured in inches, are given by the expressions…
For the manufacturing of three electronic components, protective sheets are required whose thicknesses, measured in inches, are given by the expressions $A = \sqrt[3]{5}$, $B = \sqrt[4]{7}$ and $C = \sqrt[6]{24}$. Reduce the radicals to a common index to order the thicknesses of the sheets from smallest to largest.
Perform the following operations by reducing to a common index first and extract the possible factors from the final result: a)…
Perform the following operations by reducing to a common index first and extract the possible factors from the final result:
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a) $\sqrt{2} \cdot \sqrt[3]{4} \cdot \sqrt[4]{8}$
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b) $\frac{\sqrt[4]{27}}{\sqrt[6]{3}}$
Roots of roots
Taking a root of a root means raising to a power twice. That is a power of a power, so the exponents multiply.
Express as a single radical using the root of a root property and simplify the index if possible: a) $\sqrt{\sqrt[3]{64}}$ b) $\sqrt[4]{\sqrt{x^8}}$
Express as a single radical using the root of a root property and simplify the index if possible:
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a) $\sqrt{\sqrt[3]{64}}$
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b) $\sqrt[4]{\sqrt{x^8}}$
Simplify the following expression with nested radicals by progressively introducing the factors into the innermost radicals and expressing the final result as a single…
Simplify the following expression with nested radicals by progressively introducing the factors into the innermost radicals and expressing the final result as a single root:
$$ \sqrt{2 \sqrt[3]{2 \sqrt{2}}} $$Simplify the following expression with nested radicals, reducing it to a single common index and extracting any possible factors outside the resulting root: $$ \sqrt{a…
Simplify the following expression with nested radicals, reducing it to a single common index and extracting any possible factors outside the resulting root:
$$ \sqrt{a \cdot \sqrt[3]{a^2 \cdot b \cdot \sqrt[4]{b^3}}} $$Exponent rules with rational exponents
All the exponent rules you know still hold when the exponents are fractions. This is why rational exponents are so useful: you do not need special radical rules.
| Rule | Formula |
|---|---|
| Product | $a^p \cdot a^q = a^{p+q}$ |
| Quotient | $\dfrac{a^p}{a^q} = a^{p-q}$ |
| Power of a power | $\left(a^p\right)^q = a^{pq}$ |
| Power of a product | $(ab)^p = a^p b^p$ |
| Negative exponent | $a^{-p} = \dfrac{1}{a^p}$ |
Move the slider and watch $y = x^p$. For $p = \tfrac12$ it is a square root, and for $p = 3$ it is a cube.
Operate using the properties of fractional powers and express the final result as a simplified root: $$ \frac{x^{\frac{1}{2}} \cdot \sqrt[3]{x^2}}{x^{\frac{1}{4}}} $$
Operate using the properties of fractional powers and express the final result as a simplified root:
$$ \frac{x^{\frac{1}{2}} \cdot \sqrt[3]{x^2}}{x^{\frac{1}{4}}} $$Simplify the following algebraic expression fully, assuming that $a$ and $b$ are positive real numbers. Express the final result using radical notation, without leaving…
Simplify the following algebraic expression fully, assuming that $a$ and $b$ are positive real numbers. Express the final result using radical notation, without leaving negative or fractional exponents in the solution:
$$ \left( \frac{\sqrt[4]{a^3 \cdot b} \cdot \sqrt[3]{a \cdot b^2}}{\sqrt[12]{a^5 \cdot b^7}} \right)^{-2} $$Find the value of $x$ that satisfies the following equation, first transforming all expressions into powers of base $2$ with rational exponents: $$ \sqrt[3]{16} \cdot…
Find the value of $x$ that satisfies the following equation, first transforming all expressions into powers of base $2$ with rational exponents:
$$ \sqrt[3]{16} \cdot 2^x = \frac{\sqrt{8}}{\sqrt[4]{2}} $$Express the final solution as an irreducible fraction.
Growth and fractional time
Exponential models like $N(t) = N_0 \cdot b^{t}$ do not only work for whole numbers of hours or years. With fractional exponents you can find the exact value after any fraction of a time period.
In a biology laboratory, the number of bacteria in a certain culture follows the expression $N(t) = 5000 \cdot 8^{\frac{t}{3}}$, where $t$ is the time elapsed in hours.…
In a biology laboratory, the number of bacteria in a certain culture follows the expression $N(t) = 5000 \cdot 8^{\frac{t}{3}}$, where $t$ is the time elapsed in hours. Express this formula using radicals and calculate the exact number of bacteria there will be when $2$ hours have elapsed.
The final balance $C_f$ obtained by investing an initial principal $C_0$ at an annual compound interest rate $r$ (expressed as a decimal) for $t$ years is given by the…
The final balance $C_f$ obtained by investing an initial principal $C_0$ at an annual compound interest rate $r$ (expressed as a decimal) for $t$ years is given by the formula $C_f = C_0 (1+r)^t$. A company invests $\$10000$ at an interest rate of $21$ % per annum ($r = 0,21$). Using powers with fractional exponents, calculate exactly the accumulated balance after $1$ year and $4$ months (which is equivalent to $t = \frac{4}{3}$ of a year). Knowing that $1,21 = 1,1^2$, simplify the result by extracting the possible factors from the radical.
Geometry and physics formulas
Many formulas in geometry and science contain a square root or a $\tfrac32$ power. Exact answers keep radicals, and the properties above are how you simplify them.
The diagonal of a cube joins two opposite vertices. The Pythagorean Theorem gives the diagonal of one face, $\sqrt{s^2 + s^2} = s\sqrt{2}$. Using it again with the vertical edge gives $d = \sqrt{2s^2 + s^2} = s\sqrt{3}$.
The period $T$ (in seconds) that a pendulum takes to complete one full oscillation is calculated with the physics formula $T = 2\pi \sqrt{\frac{L}{g}}$, where $L$ is the…
The period $T$ (in seconds) that a pendulum takes to complete one full oscillation is calculated with the physics formula $T = 2\pi \sqrt{\frac{L}{g}}$, where $L$ is the length of the string in feet and $g$ is the acceleration due to gravity (we will take $g = 10 \text{ ft/s}^2$). If a pendulum in a museum has a length of $L = 12$ ft, calculate its exact period by simplifying the radical fully and leaving it in terms of $\pi$.
The main diagonal of a cubic water storage tank (the straight line joining two opposite vertices passing through the center of the cube) measures exactly $\sqrt{108}$…
The main diagonal of a cubic water storage tank (the straight line joining two opposite vertices passing through the center of the cube) measures exactly $\sqrt{108}$ feet. Calculate the exact edge length of the cube and, from it, determine the capacity of the tank in cubic feet. Express all results by simplifying the radicals.
Kepler's Third Law relates the orbital period $T$ of a planet to the mean radius of its orbit $r$ using the expression $T = k^{\frac{1}{2}} \cdot r^{\frac{3}{2}}$. If…
Kepler's Third Law relates the orbital period $T$ of a planet to the mean radius of its orbit $r$ using the expression $T = k^{\frac{1}{2}} \cdot r^{\frac{3}{2}}$. If for a fictional star system the constant is $k = 4 \cdot 10^{-6}$ and a planet orbits at a mean radius of $r = 25 \cdot 10^4$ miles, calculate the exact orbital period using the properties of powers with rational exponents and radicals.