Powers with Rational Exponents. Radicals
Ximena invested $\$10{,}000$ in a promissory note and after 3 years she received $\$13{,}310$. At what constant annual rate did her money grow? To answer, you need a cube root. If you understand radicals as powers with fractional exponents, you can handle them with the same rules as powers. At the end of the page we solve Ximena's problem.
Radicals as powers with fractional exponents
Why? By the power of a power rule: $(\sqrt[n]{a})^m = (a^{1/n})^m = a^{m/n}$. Both notations represent the same number.
If the exponent is negative, you first take the reciprocal of the base, as with any power: $a^{-2/3} = \dfrac{1}{a^{2/3}} = \dfrac{1}{\sqrt[3]{a^2}}$. A decimal exponent is first converted to a fraction: $x^{1.5} = x^{3/2} = \sqrt{x^3}$.
Reducing the index
Since $\frac{4}{6}=\frac{2}{3}$, we have $\sqrt[6]{x^4}=x^{4/6}=x^{2/3}=\sqrt[3]{x^2}$. To reduce the index, you simplify the fraction of the exponent by dividing by the greatest common factor (GCF) of the index and the exponent. The value doesn't change, because the fraction is equivalent.
If the radicand is a number, break it down into powers: $\sqrt[6]{27}=\sqrt[6]{3^3}=3^{3/6}=3^{1/2}=\sqrt3$.
In the study of microorganisms, the size of certain structures can be expressed using powers and roots. Convert the following radicals into powers with fractional…
In the study of microorganisms, the size of certain structures can be expressed using powers and roots. Convert the following radicals into powers with fractional exponents and vice versa:
- a) $\sqrt[5]{x^2}$
- b) $\frac{1}{\sqrt[3]{a}}$
- c) $7^{\frac{3}{4}}$
- d) $x^{-\frac{1}{2}}$
Simplify the following radicals by dividing the index and the exponent of the radicand by their highest common factor: a) $\sqrt[12]{x^8}$ b) $\sqrt[9]{27}$ c)…
Simplify the following radicals by dividing the index and the exponent of the radicand by their highest common factor:
- a) $\sqrt[12]{x^8}$
- b) $\sqrt[9]{27}$
- c) $\sqrt[10]{a^{15}}$
- d) $\sqrt[6]{125}$
Express the following powers with rational exponents in radical form and simplify the result by taking out as many factors as possible: a) $27^{\frac{2}{3}}$ b)…
Express the following powers with rational exponents in radical form and simplify the result by taking out as many factors as possible:
- a) $27^{\frac{2}{3}}$
- b) $125^{-\frac{1}{3}}$
- c) $\left(\frac{16}{81}\right)^{\frac{3}{4}}$
- d) $x^{1.5}$
Equivalent rate in compound interest
This is where the fractional exponent becomes useful in finance: if you know the total growth over several periods, the rate per period is obtained with a root.
And now Ximena's problem: $\$10{,}000$ became $\$13{,}310$ in 3 years, so the factor is $\dfrac{13{,}310}{10{,}000}=1.331$. We have $(1+i)^3=1.331$, so $1+i=\sqrt[3]{1.331}=1.1$. The rate was $10\,\%$ per year.
Simplifying: taking out and bringing in factors
Bringing in factors is the opposite process: the factor outside is raised to the index and goes under the root. For example, $3\sqrt2=\sqrt{3^2\cdot2}=\sqrt{18}$, and $x^2\sqrt[3]{x}=\sqrt[3]{(x^2)^3\cdot x}=\sqrt[3]{x^7}$.
To optimise space in a graphic design, a programmer needs to extract factors from the radicals whenever possible. Extract factors from the radicals in the following…
To optimise space in a graphic design, a programmer needs to extract factors from the radicals whenever possible. Extract factors from the radicals in the following expressions:
- a) $\sqrt{50}$
- b) $\sqrt[3]{16x^5}$
- c) $\sqrt{a^3 b^4 c}$
- d) $\sqrt[4]{32}$
In some financial formulas, it is preferable to have all elements under a single radical sign. Insert the factors inside the root and simplify the result: a) $3\sqrt{2}$…
In some financial formulas, it is preferable to have all elements under a single radical sign. Insert the factors inside the root and simplify the result:
- a) $3\sqrt{2}$
- b) $x^2 \sqrt[3]{x}$
- c) $\frac{2}{3}\sqrt{5}$
- d) $a \sqrt[4]{a^3}$
An interior designer has planned a square rug for a law firm in Mexico City. The total area of the rug is $18 \text{ m}^2$. a) Express the length of the side of the…
An interior designer has planned a square rug for a law firm in Mexico City. The total area of the rug is $18 \text{ m}^2$.
- a) Express the length of the side of the square exactly using a radical.
- b) Simplify the radical by extracting factors.
Adding and subtracting radicals
Two radicals are like radicals if they have the same index and the same radicand. Only like radicals can be added, like like terms in algebra: $4\sqrt3+2\sqrt3=6\sqrt3$.
Sometimes radicals look different, but after taking out factors they turn out to be like radicals.
An architect is calculating the perimeters of a metal structure made up of several sections. Add and simplify the following like radicals to obtain the total length:…
An architect is calculating the perimeters of a metal structure made up of several sections. Add and simplify the following like radicals to obtain the total length:
$2\sqrt{27} - 5\sqrt{3} + \sqrt{12}$
An architectural firm is designing a triangular garden. The lengths of the three paths bordering the garden are given by the following expressions in meters: Path A:…
An architectural firm is designing a triangular garden. The lengths of the three paths bordering the garden are given by the following expressions in meters:
-
Path A: $3\sqrt{20}$
-
Path B: $\sqrt{125}$
-
Path C: $2\sqrt{45}$
Calculate the total perimeter of the garden, simplifying the expression as far as possible.
Find the value of $x$ that satisfies the following equation, simplifying all radicals beforehand: $\displaystyle \sqrt{125x} - 2\sqrt{45x} + \sqrt{500x} = 18\sqrt{5}$
Find the value of $x$ that satisfies the following equation, simplifying all radicals beforehand:
$\displaystyle \sqrt{125x} - 2\sqrt{45x} + \sqrt{500x} = 18\sqrt{5}$
Product and quotient of radicals
Examples: $\sqrt[3]{2}\cdot\sqrt[3]{4}=\sqrt[3]{8}=2$ and $\dfrac{\sqrt{18}}{\sqrt2}=\sqrt9=3$.
For the product of a sum and a radical you use the distributive property, just as with any other factor.
When the indices are different, it's best to convert everything to powers of the same base and add exponents (next section).
Perform the following multiplication and division operations with radicals having the same index, simplifying the final result: a) $\sqrt[3]{2} \times \sqrt[3]{4}$ b)…
Perform the following multiplication and division operations with radicals having the same index, simplifying the final result:
- a) $\sqrt[3]{2} \times \sqrt[3]{4}$
- b) $\frac{\sqrt{18}}{\sqrt{2}}$
- c) $\sqrt[5]{x^2} \times \sqrt[5]{x^3}$
- d) $(\sqrt[4]{a})^2$
Calculate and simplify the following expression, leaving the result as a single radical with the smallest possible index: $$\frac{\sqrt[3]{a^2} \times…
Calculate and simplify the following expression, leaving the result as a single radical with the smallest possible index:
$$\frac{\sqrt[3]{a^2} \times \sqrt[4]{a^3}}{\sqrt[6]{a^5}}$$Solve the following combined operation with radicals, expressing the result in the simplest possible form: $$\left( \sqrt{12} - \sqrt{3} \right) \times \sqrt[3]{9}$$…
Solve the following combined operation with radicals, expressing the result in the simplest possible form:
$$\left( \sqrt{12} - \sqrt{3} \right) \times \sqrt[3]{9}$$Hint: First express all terms as powers with base 3.
Different indices and radicals of radicals
The general strategy is to convert everything to powers with fractional exponents, operate with the rules for powers, and go back to the radical.
To compare radicals with different indices, you reduce them to a common index (the LCM of the indices).
If all the numbers are powers of the same base, you add exponents: $\sqrt[4]{8}\cdot\sqrt{2}=2^{3/4}\cdot2^{1/2}=2^{5/4}$. Since $2^1<2^{5/4}<2^2$, that value is between 2 and 4. With a calculator, $2^{5/4}\approx2.38$.
Simplify the following expression by reducing to a single radical and taking out factors if necessary: $$\sqrt{x \times \sqrt[3]{x^2 \times \sqrt{x}}}$$
Simplify the following expression by reducing to a single radical and taking out factors if necessary:
$$\sqrt{x \times \sqrt[3]{x^2 \times \sqrt{x}}}$$In an entrance exam, students are asked to determine which of the following two metal pieces has a greater area. Piece A is a square with side length $L_A = \sqrt[3]{4}$…
In an entrance exam, students are asked to determine which of the following two metal pieces has a greater area. Piece A is a square with side length $L_A = \sqrt[3]{4}$ cm and piece B is a square with side length $L_B = \sqrt[4]{6}$ cm.
Calculate the area of each piece and compare them by reducing the radicals to a common index.
In the design of a new microelectronic component, its resistance is given by the expression: $\displaystyle R = \sqrt[4]{27} \times \sqrt[3]{9} \times \sqrt{3}$ Express…
In the design of a new microelectronic component, its resistance is given by the expression:
$\displaystyle R = \sqrt[4]{27} \times \sqrt[3]{9} \times \sqrt{3}$
Express the radicals as powers of base 3 to find the value of $R$ as a single power with a fractional exponent. Afterwards, state which two consecutive integers this value lies between.
Combined expressions
With letters, the same thing happens: you convert everything to fractional exponents, add or subtract exponents (in a quotient they are subtracted), and at the end you write a single radical.
When there is a quotient of powers with different bases, like $\left(\dfrac{x}{y^2}\right)^{-1/2}$, the exponent is applied to each factor and the negative sign flips the fraction: $\left(\dfrac{x}{y^2}\right)^{-1/2}=\left(\dfrac{y^2}{x}\right)^{1/2}=\dfrac{y}{\sqrt x}$.
The volume of a cubic container used to transport tech goods is $V = 16\sqrt{2}$ cubic meters. a) Express the edge of the cube as a power with base 2. b) Calculate the…
The volume of a cubic container used to transport tech goods is $V = 16\sqrt{2}$ cubic meters.
- a) Express the edge of the cube as a power with base 2.
- b) Calculate the exact length of the edge in simplified radical form.
Simplify the following algebraic expression by operating with the radicals and taking out as many factors as possible. Express the final result using a single radical:…
Simplify the following algebraic expression by operating with the radicals and taking out as many factors as possible. Express the final result using a single radical:
$\displaystyle \frac{\sqrt{x^3 \times \sqrt[3]{x^2 \times y}}}{\sqrt[6]{x \times y^5}} \times \left( \frac{x}{y^2} \right)^{-1/2}$
Simplify the following combined expression, leaving the result in its simplest form:…
Simplify the following combined expression, leaving the result in its simplest form:
$\displaystyle \frac{\left( \sqrt[3]{a^2 \times b} \right)^2 \times \sqrt{a \times b^3}}{\sqrt[6]{a^5 \times b}}$
Applications in geometry
Radicals appear when you solve for a side in an area or volume formula, or with the Pythagorean theorem. It's best to leave the result exact, with simplified radicals, and use the calculator only at the end.
The diagonal of a cube with edge $a$ measures $a\sqrt3$. If the diagonal is $\sqrt{75}=5\sqrt3$, then $a=5$: the edge turns out to be rational even though the diagonal is irrational.
Calculate the exact total surface area of a regular octahedron (formed by two square-based pyramids joined at their base) whose edge length is $\sqrt{6} + \sqrt{2}$ cm.…
Calculate the exact total surface area of a regular octahedron (formed by two square-based pyramids joined at their base) whose edge length is $\sqrt{6} + \sqrt{2}$ cm. You must leave the result simplified in radical form, without using decimal approximations.
(Hint: The area of an equilateral triangle of side length $a$ is $A = \frac{\sqrt{3}}{4}a^2$)
An architect is designing a right triangular window. The hypotenuse measures $\sqrt[4]{1024}$ meters and one of the shorter sides measures $\sqrt{2}$ meters. a)…
An architect is designing a right triangular window. The hypotenuse measures $\sqrt[4]{1024}$ meters and one of the shorter sides measures $\sqrt{2}$ meters.
- a) Calculate the exact length of the other side in simplified radical form.
- b) Calculate the area of the window. Is the area a rational or irrational number? Justify your answer.
A cubic water tank is to be constructed with an interior diagonal measuring exactly $\sqrt{108}$ meters. a) Show that the edge length of the tank is a rational number.…
A cubic water tank is to be constructed with an interior diagonal measuring exactly $\sqrt{108}$ meters.
- a) Show that the edge length of the tank is a rational number.
- b) If the cost of waterproofing the interior walls (the 4 side faces and the floor) is $3\,000 \times \sqrt{3}$ pesos per square meter, calculate the exact total cost of the work.
Rationalizing denominators
To rationalize is to write a fraction so that the denominator has no roots. You multiply the numerator and denominator by the same thing, so the value doesn't change.
Case 1: a lone square root. You multiply by the same root: $\dfrac{5}{\sqrt3}=\dfrac{5\sqrt3}{\sqrt3\cdot\sqrt3}=\dfrac{5\sqrt3}{3}$. Also $\dfrac{x}{\sqrt x}=\dfrac{x\sqrt x}{x}=\sqrt x$.
Case 2: a root with a different index. You complete the exponent until it equals the index: $\dfrac{1}{\sqrt[3]{2}}=\dfrac{\sqrt[3]{2^2}}{\sqrt[3]{2}\cdot\sqrt[3]{2^2}}=\dfrac{\sqrt[3]{4}}{2}$.
Case 3: a sum or difference of roots. You multiply by the conjugate (the same binomial with the middle sign changed) and use $(a+b)(a-b)=a^2-b^2$.
Case 4: three terms. You group two of them to form a binomial and rationalize in two steps.
Rationalise the following expressions to eliminate the radicals from the denominator, simplifying the result where possible: a) $\frac{5}{\sqrt{3}}$ b)…
Rationalise the following expressions to eliminate the radicals from the denominator, simplifying the result where possible:
- a) $\frac{5}{\sqrt{3}}$
- b) $\frac{1}{\sqrt[3]{2}}$
- c) $\frac{\sqrt{2}}{\sqrt{5}}$
- d) $\frac{x}{\sqrt{x}}$
Rationalise and simplify the following expressions so that they do not contain roots in the denominator: a) $\frac{10}{\sqrt[3]{5^2}}$ b)…
Rationalise and simplify the following expressions so that they do not contain roots in the denominator:
- a) $\frac{10}{\sqrt[3]{5^2}}$
- b) $\frac{2\sqrt{3}}{\sqrt{7} - \sqrt{2}}$
- c) $\frac{\sqrt{5} + \sqrt{3}}{\sqrt{5} - \sqrt{3}}$
Rationalise and simplify the following expression as far as possible. Ensure that the final denominator is an integer:…
Rationalise and simplify the following expression as far as possible. Ensure that the final denominator is an integer:
$\displaystyle \frac{2\sqrt{3}}{\sqrt{2} + \sqrt{3} - \sqrt{5}}$