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ESO Y4B · Mathematics
Numbers · Powers, roots and logarithms

Properties of radicals and rationalising the denominator

A result such as $\dfrac{\sqrt2}{\sqrt3+\sqrt2}$ is not a convenient answer to keep working with, and $5\sqrt3 - 2\sqrt3$ can be reduced to a single term. This section brings together the techniques for leaving any expression with radicals in its simplest form.

Before you start, make sure you know: Notable identities (2nd ESO)
Before you start, make sure you know: Taking out factors (3rd ESO)

Simplifying: taking out factors and reducing the index

Step-by-step diagram of taking factors out of the square root of 72 by writing it as 36 times 2 until reaching 6 root 2 √72 perfect square: 36 · 2 = √(36·2) √36 = 6 comes out of the root = 6√2 36 is the largest perfect square that divides 72
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Take out as many factors as possible from the following radicals: a) $\sqrt{72}$ b) $\sqrt[3]{250}$

Take out as many factors as possible from the following radicals:

  1. a) $\sqrt{72}$
  2. b) $\sqrt[3]{250}$
Solve
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Simplify the following radicals by dividing the index and the exponent by their highest common factor: a) $\sqrt[6]{27}$ b) $\sqrt[8]{16}$

Simplify the following radicals by dividing the index and the exponent by their highest common factor:

  1. a) $\sqrt[6]{27}$
  2. b) $\sqrt[8]{16}$
Solve
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Express as a single radical and simplify the result: a) $\sqrt{\sqrt[3]{64}}$ b) $\left(\sqrt[3]{\sqrt{2}}\right)^6$

Express as a single radical and simplify the result:

  1. a) $\sqrt{\sqrt[3]{64}}$
  2. b) $\left(\sqrt[3]{\sqrt{2}}\right)^6$
Solve

Adding and subtracting like radicals

Two radicals are like radicals if they have the same index and the same radicand. They can only be added or subtracted if they are like radicals, just as with like terms in algebra.

Step-by-step diagram of adding like radicals by taking factors out of root 50 and root 18 to reduce them to root 2 3√50 + 2√18 − √8 = 3·5√2 + 2·3√2 − 2√2 = 15√2 + 6√2 − 2√2 = 19√2
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Perform the following additions and subtractions of like radicals: a) $5\sqrt{3} - 2\sqrt{3} + 4\sqrt{3}$ b) $\frac{1}{2}\sqrt{7} + \frac{3}{2}\sqrt{7} - \sqrt{7}$

Perform the following additions and subtractions of like radicals:

  1. a) $5\sqrt{3} - 2\sqrt{3} + 4\sqrt{3}$
  2. b) $\frac{1}{2}\sqrt{7} + \frac{3}{2}\sqrt{7} - \sqrt{7}$
Solve
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Perform the following operation, first taking out the necessary factors so that the radicals are like terms: $3\sqrt{50} + 2\sqrt{18} - \sqrt{8}$

Perform the following operation, first taking out the necessary factors so that the radicals are like terms:

$3\sqrt{50} + 2\sqrt{18} - \sqrt{8}$

Solve

Multiplying and dividing with different indices

Diagram of multiplying the square root of 2 by the cube root of 3 by reducing to common index 6 using fractional exponents √2 · ∛3 they are written as powers = 23/6 · 32/6 common denominator 6: a single radical = ⁶√(2³·3²) same index (6): they can now be combined

With the same index you operate directly on the radicands; with different indices, you first have to reduce to a common index (convert the fractions in the exponents to the same denominator).

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Perform the following multiplications of radicals with the same index and take out factors if possible: a) $\sqrt{2} \cdot \sqrt{8}$ b) $\sqrt[3]{4} \cdot \sqrt[3]{10}$

Perform the following multiplications of radicals with the same index and take out factors if possible:

  1. a) $\sqrt{2} \cdot \sqrt{8}$
  2. b) $\sqrt[3]{4} \cdot \sqrt[3]{10}$
Solve
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Reduce to a common index and perform the following multiplication and division of radicals: $\frac{\sqrt{2} \cdot \sqrt[3]{3}}{\sqrt[6]{6}}$

Reduce to a common index and perform the following multiplication and division of radicals:

$\frac{\sqrt{2} \cdot \sqrt[3]{3}}{\sqrt[6]{6}}$

Solve
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Simplify the following expression by inserting the factors into the innermost radical and expressing it as a single root: $\sqrt{2 \cdot \sqrt[3]{2}}$

Simplify the following expression by inserting the factors into the innermost radical and expressing it as a single root:

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

Solve

Rationalising denominators

Leaving a root in the denominator is not "wrong", but it is not the standard form: we rationalise by multiplying top and bottom by whatever is needed so that the denominator is left without roots.

Step-by-step diagram of rationalising a binomial denominator by multiplying by its conjugate to apply a difference of two squares 6√3 · √3√3 = 6√33 = 2√3 monomial denominator: multiply by the same root top and bottom 4√5−1 · √5+1√5+1 multiply by the conjugate: (√5−1)(√5+1) = 5−1 = 4
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Rationalise the denominator of the following fractions and simplify the result: a) $\frac{6}{\sqrt{3}}$ b) $\frac{10}{\sqrt{5}}$

Rationalise the denominator of the following fractions and simplify the result:

  1. a) $\frac{6}{\sqrt{3}}$
  2. b) $\frac{10}{\sqrt{5}}$
Solve
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Rationalise the following expressions by multiplying by the conjugate of the denominator: a) $\frac{4}{\sqrt{5}-1}$ b) $\frac{2}{\sqrt{3}+1}$

Rationalise the following expressions by multiplying by the conjugate of the denominator:

  1. a) $\frac{4}{\sqrt{5}-1}$
  2. b) $\frac{2}{\sqrt{3}+1}$
Solve
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Rationalise the following fractions with roots of index greater than 2: a) $\frac{10}{\sqrt[5]{8}}$ b) $\frac{3}{\sqrt[4]{27}}$

Rationalise the following fractions with roots of index greater than 2:

  1. a) $\frac{10}{\sqrt[5]{8}}$
  2. b) $\frac{3}{\sqrt[4]{27}}$
Solve
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Rationalise the denominator of the following fraction and simplify as much as possible by working out the numerator: $\frac{\sqrt{6}-\sqrt{2}}{\sqrt{6}+\sqrt{2}}$

Rationalise the denominator of the following fraction and simplify as much as possible by working out the numerator:

$\frac{\sqrt{6}-\sqrt{2}}{\sqrt{6}+\sqrt{2}}$

Solve

Applications in geometry and physics

Isosceles triangle with equal sides of root 15, base root 6 and height h drawn with a dashed line b = √6 √15 √15 h
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In a carpentry workshop, a rectangular board with exact dimensions of $\sqrt{12}$ dm wide and $\sqrt{27}$ dm long is going to be cut. Calculate the area and perimetre of…

In a carpentry workshop, a rectangular board with exact dimensions of $\sqrt{12}$ dm wide and $\sqrt{27}$ dm long is going to be cut. Calculate the area and perimetre of the board, expressing the results in exact form using simplified radicals.

Solve
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Calculate the exact length of the main diagonal $D$ of a cube with an edge length of $a = \sqrt{5}$ cm. Remember that the diagonal of a cube is calculated as…

Calculate the exact length of the main diagonal $D$ of a cube with an edge length of $a = \sqrt{5}$ cm. Remember that the diagonal of a cube is calculated as $D = \sqrt{a^2 + a^2 + a^2}$. Express the result in the most simplified form possible.

D = ? a a a
Solve
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The front wall of a cabin is in the shape of an isosceles triangle. The base of the triangle measures $\sqrt{6}$ metres and the two equal sides measure $\sqrt{15}$…

The front wall of a cabin is in the shape of an isosceles triangle. The base of the triangle measures $\sqrt{6}$ metres and the two equal sides measure $\sqrt{15}$ metres each. Calculate the exact value of the height $h$ perpendicular to the base and the total area of the front wall. Rationalise the results if roots appear in the denominator.

b = √6 √15 √15 h
Solve
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In physics, the speed $v$ of an object can be rearranged from its kinetic energy $E_c$ and its mass $m$ using the formula $v = \sqrt{\frac{2E_c}{m}}$. If a prototype…

In physics, the speed $v$ of an object can be rearranged from its kinetic energy $E_c$ and its mass $m$ using the formula $v = \sqrt{\frac{2E_c}{m}}$. If a prototype vehicle has a mass of $m = \sqrt{3}$ kg and its kinetic energy is $E_c = 45$ J, calculate the exact value of its speed. Rationalise and take out any possible factors from the resulting root.

Solve

Back to the beginning: Rationalising with the conjugate gives $\dfrac{\sqrt2}{\sqrt3+\sqrt2} = \dfrac{\sqrt2(\sqrt3-\sqrt2)}{(\sqrt3+\sqrt2)(\sqrt3-\sqrt2)} = \dfrac{\sqrt6-2}{3-2} = \sqrt6 - 2$, and the subtraction of like radicals gives $5\sqrt3 - 2\sqrt3 = 3\sqrt3$.

Summary

See all the exercises in this section