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ESO Y4B · Mathematics
Numbers · Powers, roots and logarithms

Powers with rational exponents. Radicals

The period of a pendulum, Kepler's third law, the speed of a wave: many science formulae contain a root, and handling them by hand (without a calculator, and exactly) requires seeing them for what they really are: powers with an exponent that is not a whole number.

Before you start, make sure you know: Operations with powers (3rd ESO)
Before you start, make sure you know: Roots (3rd ESO)

From radical to fractional power

Diagram showing the conversion of the fourth root of x to the fifth into a fractional power: the index becomes the denominator and the exponent becomes the numerator. ∜(x5) index 4 = x 5 4 index of the root → denominator of the exponent exponent inside → numerator of the exponent
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Express the following powers with fractional exponents as radicals 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 radicals and calculate their exact value if it is an integer:

  • a) $27^{\frac{1}{3}}$

  • b) $16^{\frac{3}{4}}$

  • c) $5^{\frac{2}{3}}$

Solve
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Write the following radicals in the form of a power with a fractional exponent and simplify the fraction in 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 in the exponent:

  • a) $\sqrt[6]{x^2}$

  • b) $\sqrt[15]{a^5}$

  • c) $\sqrt[8]{2^6}$

Solve

Taking factors out and putting them in

Converting to a fractional exponent lets you use the laws of indices instead of manipulating roots blindly; in practice, to calculate a value it is best to take out first any factors you can.

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Take out all possible factors from inside the following radicals: a) $\sqrt{50}$ b) $\sqrt[3]{16}$ c) $\sqrt[4]{x^7 y^5}$

Take out all possible factors from inside the following radicals:

  • a) $\sqrt{50}$

  • b) $\sqrt[3]{16}$

  • c) $\sqrt[4]{x^7 y^5}$

Solve
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Insert the factors into 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 $a \ge 0$

Insert the factors into 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 $a \ge 0$

Solve

Multiplying, dividing and raising radicals of the same index

With the same index, radicals are multiplied and divided by operating on the radicands; to raise a radical to a power, the exponent goes inside.

Interactive or visual diagram showing the procedure for taking factors out of a radical by separating powers whose exponents are multiples of the index. √3 · √12 same index: a single root = √(3·12) the radicands are multiplied = √36 = 6 and then simplified
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Calculate the following products of radicals of the same index and take out factors from the result if possible: a) $\sqrt{3} \cdot \sqrt{12}$ b)…

Calculate the following products of radicals of the same index and take out factors from the result if possible:

  • a) $\sqrt{3} \cdot \sqrt{12}$

  • b) $\sqrt[3]{4} \cdot \sqrt[3]{10}$

Solve
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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:

  • a) $\frac{\sqrt{72}}{\sqrt{2}}$

  • b) $\frac{\sqrt[3]{40}}{\sqrt[3]{5}}$

Solve
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Calculate the following powers of radicals, moving the exponent inside the radicand and expressing the result in the simplest form possible: a) $(\sqrt[3]{5})^6$ b)…

Calculate the following powers of radicals, moving the exponent inside the radicand and expressing the result in the simplest form possible:

  • a) $(\sqrt[3]{5})^6$

  • b) $(\sqrt{x^3})^4$

Solve

Common index and nested radicals

If the indices are different, you cannot operate directly: you must rewrite the fractions in the exponents until they have the same denominator.

Multiplication of the square root of x by the cube root of x by converting them into fractional powers and adding the fractions over the common denominator six. √x · ∛x = x1/2 · x1/3 = x3/6+2/6 1/2 = 3/6 and 1/3 = 2/6: now they can be added = x5/6
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Express as a single radical using the root of a root rule 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 rule and simplify the index if possible:

  • a) $\sqrt{\sqrt[3]{64}}$

  • b) $\sqrt[4]{\sqrt{x^8}}$

Solve
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For the manufacture of three electronic components, protective sheets are required whose thicknesses, measured in millimetres, are given by the expressions…

For the manufacture of three electronic components, protective sheets are required whose thicknesses, measured in millimetres, are given by the expressions $A = \sqrt[3]{5}$, $B = \sqrt[4]{7}$ and $C = \sqrt[6]{24}$. Express the radicals with a common index to order the sheet thicknesses from smallest to largest.

Solve
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Simplify the following expression with nested radicals by progressively moving factors into the innermost radicals and expressing the final result as a single root: $$…

Simplify the following expression with nested radicals by progressively moving factors into the innermost radicals and expressing the final result as a single root:

$$ \sqrt{2 \sqrt[3]{2 \sqrt{2}}} $$
Solve

Applications in science and geometry

Graph of the orbital period T against the orbital radius r according to Kepler's third law, T = k to the power 1/2 times r to the power 3/2: a power with a fractional exponent whose slope always increases. T (period)r (orbital radius) T = k¹ᐟ² · r³ᐟ²
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In a biology laboratory, the number of bacteria in a given 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 given 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 present after $2$ hours.

Solve
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A circular town square has an area of $120\pi$ m$^2$. Given that the area of a circle is given by the formula $A = \pi r^2$, calculate the exact radius of the square,…

A circular town square has an area of $120\pi$ m$^2$. Given that the area of a circle is given by the formula $A = \pi r^2$, calculate the exact radius of the square, fully simplifying the radical.

Solve
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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 the constant for a fictional star system is $k = 4 \cdot 10^{-6}$ and a planet orbits at a mean radius of $r = 25 \cdot 10^4$ km, calculate the exact orbital period using the properties of powers with rational exponents and radicals.

Solve
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In maritime navigation, the distance $d$ (in kilometres) to the visible horizon for an observer situated at a height $h$ (in metres) above sea level is approximated by…

In maritime navigation, the distance $d$ (in kilometres) to the visible horizon for an observer situated at a height $h$ (in metres) above sea level is approximated by the formula $d = \sqrt{13h}$. Two observers are at different points on a cliff at heights of $h_1 = 52$ m and $h_2 = 117$ m respectively. Calculate the exact difference between their distances to the visible horizon, simplifying the expression $\sqrt{13 \cdot 117} - \sqrt{13 \cdot 52}$ by taking out factors.

Solve

Summary

See all the exercises in this section