Rational Exponents and Radicals
Aaliyah puts $\$500$ into an account that grows 44% each year. After 1.5 years her balance is $500 \cdot 1.44^{3/2}$. But what does it even mean to raise a number to the $\tfrac{3}{2}$ power? And can you find the exact value without a calculator? By the end of this page you will.
Radicals as Rational Exponents
You already know that $9^1 \cdot 9^1 = 9^2$: when you multiply powers with the same base, you add the exponents. Now ask: what number $9^{1/2}$ would satisfy $9^{1/2} \cdot 9^{1/2} = 9^{1/2 + 1/2} = 9^1$? A number that times itself gives $9$, which is $\sqrt{9} = 3$. So it makes sense to define $9^{1/2} = \sqrt{9}$. The same idea works for every index.
Because $a^{m/n} = (a^{1/n})^m$, you can take the root first and then the power. For exact work, always take the root first: the numbers stay small.
The graphs above are the square root and cube root curves. Notice that both pass through $(0, 0)$ and $(1, 1)$.
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: Pulling Factors Out and Putting Them In
A radical is in simplest form when no factor of the radicand is a perfect power of the index. To simplify, look for the largest perfect square (for $\sqrt{\ }$), perfect cube (for $\sqrt[3]{\ }$), and so on, that divides the radicand.
Why does it work? A root of a product is the product of the roots, so $\sqrt{25 \cdot 2} = \sqrt{25}\cdot\sqrt{2} = 5\sqrt{2}$.
With variables, divide each exponent by the index. The quotient is how many copies come out of the radical, and the remainder stays inside.
The reverse move is putting a factor inside the radical. A factor $a$ outside an index-$n$ radical goes in as $a^n$:
$$a\sqrt[n]{b} = \sqrt[n]{a^n \cdot b}$$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 $a \ge 0$
Multiplying, Dividing and Raising Radicals with the Same Index
When the indices match, you can work on the radicands directly.
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) $(\sqrt{x^3})^4$
Adding Radicals and Rationalizing Denominators
Radicals add like terms do. Two radicals with the same index and the same radicand are like radicals, and you add their coefficients: $4\sqrt{3} + 5\sqrt{3} = 9\sqrt{3}$.
Radicals with different radicands cannot be combined: $\sqrt{2} + \sqrt{3} \neq \sqrt{5}$. But radicals that don't look alike may become alike once you simplify them.
By convention, a simplified answer has no radical in the denominator. To get rid of it, multiply the numerator and denominator by the radical in the denominator. This is called rationalizing the denominator.
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.
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.
Common Index and Nested Radicals
If the indices are different, you can't combine the radicands directly. Write each radical as a power and give the exponents a common denominator.
The same trick lets you compare radicals: give them the same index, then compare the radicands.
For nested radicals that also have factors in front, work from the inside out: rewrite the inner radical as a power, multiply it into its radicand, then handle the next layer.
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) $\sqrt[4]{\sqrt{x^8}}$
For the manufacture of three electronic components, protective sheets are required whose thicknesses, measured in inches, are given by the expressions $A = \sqrt[3]{5}$,…
For the manufacture 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.
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:
$$ \sqrt{2 \sqrt[3]{2 \sqrt{2}}} $$Exponent Rules with Rational Exponents
Every rule you know for integer exponents works for rational exponents too. That is the main reason to convert radicals into powers: you only need one set of rules.
Two more facts you need: $(ab)^p = a^p b^p$ and $a^{-p} = \dfrac{1}{a^p}$. To combine fractional exponents, use a common denominator, as with any fractions.
When you want a final answer in radical form, write it with positive exponents and no fractional exponents left.
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) $\frac{\sqrt[4]{27}}{\sqrt[6]{3}}$
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:
$$ \left( \frac{\sqrt[4]{a^3 \cdot b} \cdot \sqrt[3]{a \cdot b^2}}{\sqrt[12]{a^5 \cdot b^7}} \right)^{-2} $$Powers as a Problem-Solving Tool
Messy problems (nested radicals with variables, equations with radicals, proofs) all yield to the same plan: turn everything into powers of the same base.
Equations. If two powers of the same base are equal, their exponents are equal: $2^u = 2^v$ means $u = v$. So rewrite both sides as powers of one base, then solve the linear equation for the exponent. Some problems ask for the answer as a fraction in simplest form (lowest terms), so reduce your final fraction.
Proofs. To show that two radical expressions are equal, convert both to powers and show the exponents match. Each equal sign must be justified by a rule.
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}} $$Express the final solution as an irreducible fraction.
Prove, rigorously using the properties of powers with rational exponents, that for any positive real number $x$, the expression $\sqrt[n]{\sqrt[m]{x}}$ is equivalent to…
Prove, rigorously using the properties of powers with rational exponents, that for any positive real number $x$, the expression $\sqrt[n]{\sqrt[m]{x}}$ is equivalent to $\sqrt[m]{\sqrt[n]{x}}$. Then, apply this property or convert to powers to calculate the exact numerical value of the following sum:
$$ \sqrt[3]{\sqrt{64}} + \sqrt{\sqrt[3]{729}} $$Simplify the following expression with nested radicals, reducing it to a single common index and extracting any possible factors outside the resulting root: $$ \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:
$$ \sqrt{a \cdot \sqrt[3]{a^2 \cdot b \cdot \sqrt[4]{b^3}}} $$Applications: Growth and Geometry
Formulas from science, finance and geometry are full of fractional exponents. Convert, take the root first, and keep the answer exact.
Back to Aaliyah. Compound interest uses $A = P(1 + r)^t$, and $t$ can be a fraction. Since $1.44 = 1.2^2$, we get $1.44^{3/2} = \left(1.2^2\right)^{3/2} = 1.2^3 = 1.728$. Her balance is $500 \cdot 1.728 = \$864$, exactly, with no calculator. A power with exponent $\tfrac{3}{2}$ simply means "square root, then cube."
When the exponent is not a whole number of roots, rewrite the base as a perfect power and leave the leftover as a radical, pulling out whole factors.
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 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.
The final capital $C_f$ obtained by investing an initial capital $C_0$ at an annual compound interest rate $r$ (expressed as a decimal) for $t$ years is given by the…
The final capital $C_f$ obtained by investing an initial capital $C_0$ 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 year ($r = 0,21$). Using powers with fractional exponents, calculate exactly the accumulated capital 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.
Radicals in Formulas: Exact Values with π and Powers of 10
Physics formulas often combine a radical with $\pi$, with scientific notation, or with a sum of square roots. The same tools work. Two reminders make them painless.
$\pi$ is just a number that stays outside. You don't need to replace it with $3.14$. Simplify the radical, and write the answer as a coefficient times $\pi$.
Split powers of 10 by evenness. For a power of 10 with a rational exponent, apply the exponent to each factor: $(a \cdot 10^n)^{p} = a^p \cdot 10^{np}$. Rewrite $a$ as a perfect power when you can.
The period $T$ (in seconds) that a pendulum takes to complete one full oscillation is calculated with the physical formula $T = 2\pi \sqrt{\frac{L}{g}}$, where $L$ is…
The period $T$ (in seconds) that a pendulum takes to complete one full oscillation is calculated with the physical 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 $\pi$.
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.
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.