Square roots & Cube roots
🎯 In this topic you will
- Understand how square and cube numbers relate to square roots and cube roots
- Find the squares of positive and negative integers and their corresponding square roots
- Find the cubes of positive and negative integers and their corresponding cube roots
- Recognise natural numbers, integers, and rational numbers
🧠 Key Words
- cube number
- cube root
- consecutive
- equivalent
- index
- natural numbers
- rational numbers
- square number
- square root
Show Definitions
- cube number: A number made by multiplying a whole number by itself twice (e.g., 2 × 2 × 2 = 8).
- cube root: A number that when multiplied by itself twice gives the original number (e.g., ∛8 = 2).
- consecutive: Numbers that follow one after another in order without gaps.
- equivalent: Having the same value or amount, even if they look different (e.g., 1/2 = 2/4).
- index: A number that shows how many times a base number is multiplied by itself (also called an exponent).
- natural numbers: The counting numbers starting from 1, 2, 3, and so on.
- rational numbers: Numbers that can be written as a fraction or ratio of two integers.
- square number: A number made by multiplying a whole number by itself (e.g., 3 × 3 = 9).
- square root: A number that when multiplied by itself gives the original number (e.g., √9 = 3).
🟣 Square numbers and square roots
1 × 1 = 1 2 × 2 = 4 3 × 3 = 9 4 × 4 = 16 5 × 5 = 25
The square numbers are 1, 4, 9, 16, 25, …
You use an index of 2 to show square numbers.
12 = 1 22 = 4 32 = 9 42 = 16 52 = 25
You read 12 as ‘1 squared’ and you read 22 as ‘2 squared’.
42 = 16 is equivalent to 4 = √16, which is read as ‘4 is the square root of 16’.
The symbol for square root is √.
🔎 Reasoning Tip
Cube root symbol: The symbol for cube root is ∛.
🟠 Cube numbers and cube roots
1 × 1 × 1 = 1 2 × 2 × 2 = 8 3 × 3 × 3 = 27
The cube numbers are 1, 8, 27, …
You use an index of 3 and write 13 = 1, 23 = 8, 33 = 27, …
You read 13 as ‘1 cubed’ and you read 23 as ‘2 cubed’.
You say ‘2 is the cube root of 8’, which is written as 2 = ∛8.
🧠 Estimating square roots
You can estimate the square roots of integers that are not square numbers.
❓ EXERCISES
1a. Copy and complete: $3^2 =$
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1b. Copy and complete: $5^2 =$
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1c. Copy and complete: $8^2 =$
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1d. Copy and complete: $10^2 =$
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1e. Copy and complete: $15^2 =$
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2. An equivalent statement to $7^2 = 49$ is $\sqrt{49} = 7$. Write equivalent statements to your answers to Question 1.
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$\sqrt{25} = 5$
$\sqrt{64} = 8$
$\sqrt{100} = 10$
$\sqrt{225} = 15$
3a. Find $\sqrt{36}$
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3b. Find $\sqrt{81}$
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3c. Find $\sqrt{121}$
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3d. Find $\sqrt{144}$
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4a. $1^3 =$
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4b. $2^3 =$
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4c. $3^3 =$
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4d. $4^3 =$
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4e. $5^3 =$
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5. Write equivalent statements to your answers to Question 4, using cube root notation.
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$\sqrt[3]{8} = 2$
$\sqrt[3]{27} = 3$
$\sqrt[3]{64} = 4$
$\sqrt[3]{125} = 5$
6a. Work out the integer closest to $\sqrt{15}$
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6b. Work out the integer closest to $\sqrt{66}$
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6c. Work out the integer closest to $\sqrt{150}$
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7a. Show that $\sqrt{90}$ is between 9 and 10.
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Therefore, $\sqrt{90}$ is between 9 and 10.
7b. Find two consecutive integers to complete: $\sqrt{180}$ is between … and …
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So $\sqrt{180}$ is between 13 and 14.
7c. Find two consecutive integers to complete: $\sqrt[3]{90}$ is between … and …
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So $\sqrt[3]{90}$ is between 4 and 5.
8a. Use a calculator to find $17^2$.
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8b. Complete this statement: $\sqrt{\,\boxed{\phantom{0}}\,} = 17$
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9a. $\sqrt[3]{\boxed{\phantom{0}}} = 18$
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9b. $\sqrt[3]{\boxed{\phantom{0}}} = 20$
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9c. $\sqrt[3]{\boxed{\phantom{0}}} = 23$
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9d. $\sqrt[3]{\boxed{\phantom{0}}} = 26$
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10a. $\sqrt[3]{\boxed{\phantom{0}}} = 7$
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10b. $\sqrt[3]{\boxed{\phantom{0}}} = 9$
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10c. $\sqrt[3]{\boxed{\phantom{0}}} = 10$
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10d. $\sqrt[3]{\boxed{\phantom{0}}} = 12$
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11a. Show that 36 has nine factors.
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1, 2, 3, 4, 6, 9, 12, 18, 36 → 9 factors
11b(i). Find the factors of 9.
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11b(ii). Find the factors of 16.
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11b(iii). Find the factors of 25.
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11c. Explain why every square number has an odd number of factors.
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11d. Find a number that is not square and has an odd number of factors.
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Only perfect squares have an odd number of factors.
11e. Does every cube number have an odd number of factors? Give a reason.
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11f. Investigate how many factors different square numbers have.
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- $4$ has 3 factors: 1, 2, 4
- $9$ has 3 factors: 1, 3, 9
- $16$ has 5 factors: 1, 2, 4, 8, 16
- $25$ has 3 factors: 1, 5, 25
- $36$ has 9 factors: 1, 2, 3, 4, 6, 9, 12, 18, 36
🧠 Think like a Mathematician
Question: What patterns can you find in the differences between consecutive square numbers and consecutive cube numbers?
Equipment: Pencil, paper, calculator (optional)
- Note: $1^2 = 1$ and $2^2 = 4$, so the difference is $4 - 1 = 3$.
- a) Copy and complete this diagram to find the differences between consecutive square numbers:
Square numbers:$1^2$, $2^2$, $3^2$, $4^2$, $5^2$, $6^2$
Differences:3, 5, 7, 9, 11
- b) Describe any pattern in your answers.
- c) Investigate the differences between consecutive cube numbers:
Cube numbers:$1^3$, $2^3$, $3^3$, $4^3$, $5^3$, $6^3$
Differences:7, 19, 37, 61, 91
Follow-up Questions:
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- 1: The differences between consecutive square numbers increase by 2 each time: $3, 5, 7, 9, 11...$
- 2: The differences between consecutive cube numbers increase in a quadratic pattern: $7, 19, 37, 61, 91...$
- 3: The difference between consecutive squares is always an odd number. For cubes, the differences follow a pattern that increases more rapidly — the second differences are constant (i.e., the differences between the differences are equal).
❓ EXERCISES
12a(i). Work out: $\sqrt{1^3}$
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12a(ii). Work out: $\sqrt{1^3 + 2^3}$
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12a(iii). Work out: $\sqrt{1^3 + 2^3 + 3^3}$
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12b. What do you notice about your answers to part a?
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$\sqrt{1^3 + 2^3} = 3$, and $1 + 2 = 3$.
$\sqrt{1^3 + 2^3 + 3^3} = 6$, and $1 + 2 + 3 = 6$.
12c. Does the pattern continue with more cubes? Explain.
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$1^3 + 2^3 + 3^3 + \dots + n^3 = \left( \dfrac{n(n+1)}{2} \right)^2$
13a. Add the first three odd numbers and find the square root of the answer.
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13b. Add the first four odd numbers and find the square root of the answer.
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13c. Can you generalise the results of parts a and b?
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13d. Look at this diagram. How is it connected to the earlier parts?

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This shows that adding consecutive odd numbers builds perfect squares — just like in parts a–c.
⚠️ Be careful!
Don’t confuse $\sqrt{64}$ with $\sqrt[3]{64}$. The square root of 64 is $8$, but the cube root of 64 is $4$ — they are different operations with different results!
🍬 Learning Bridge
Now that you've explored square and cube numbers — and learned how to estimate their roots — you're ready to go deeper. You'll investigate what happens when roots involve negative numbers, discover why square roots can have two answers, and learn how these ideas help solve real equations.
🔁 Positive and negative roots
52 = 25
This means that the square root of 25 is 5. This can be written as $\sqrt{25} = 5$.
This is the only answer in the set of natural numbers.
However (–5)2 = –5 × –5 = 25
This means that the integer –5 is also a square root of 25.
Every positive integer has two square roots, one positive and one negative.
5 is the positive square root of 25 and –5 is the negative square root.
No negative number has a square root.
For example, the integer –25 has no square root because the equation $x^2 = -25$ has no solution.
53 = 125
This means that the cube root of 125 is 5. This can be written as $\sqrt[3]{125} = 5$.
You might think –5 is also a cube root of 125.
However (–5)3 = –5 × –5 × –5 = (–5 × –5) × –5 = 25 × –5 = –125
So $\sqrt[3]{-125} = -5$
Every number, positive or negative or zero, has only one cube root.
🔎 Reasoning Tip
Natural numbers: These are the counting numbers and zero.
❓ EXERCISES
14. Work out
a) $7^2$
b) $(-7)^2$
c) $7^3$
d) $(-7)^3$
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b) 49
c) 343
d) –343
15. Find
a) $\sqrt[3]{125}$
b) $\sqrt[3]{-27}$
c) $\sqrt[3]{1}$
d) $\sqrt[3]{8}$
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b) –3
c) 1
d) 2
16. Solve these equations:
a) $x^2 = 100$
b) $x^2 = 144$
c) $x^2 = 1$
d) $x^2 = 0$
e) $x^2 + 9 = 0$
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b) $x = ±12$
c) $x = ±1$
d) $x = 0$
e) No real solution (complex roots)
17. Solve these equations:
a) $x^3 = 216$
b) $x^3 + 27 = 0$
c) $x^3 + 1 = 0$
d) $x^3 + 125 = 0$
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b) $x = -3$
c) $x = -1$
d) $x = -5$
18. Use the fact $27^2 = 9^3 = 729$ to find:
a) $\sqrt{729}$
b) $\sqrt{-729}$
c) $\sqrt[3]{729}$
d) $\sqrt[3]{-729}$
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b) Not a real number
c) 9
d) –9
19. a) A calculator shows that $\sqrt{82} - \sqrt{(-8)^2} = 0$.
Explain why this is correct.
b) Find the value of $\sqrt[3]{4^3} - \sqrt[3]{(-4)^3}$. Show your working.
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So both values are approximately the same.
b) $4^3 = 64$ and $(-4)^3 = -64$
So $\sqrt[3]{64} - \sqrt[3]{-64} = 4 - (-4) = 8$
20. The square of an integer is 100.
What can you say about the cube of the integer?
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21. The integer 1521 = $3^2 \times 13^2$
Use this fact to:
a) Find $\sqrt{1521}$
b) Solve the equation $x^2 = 1521$
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b) $x = ±39$
22. a) How is $-5^2$ different from $(-5)^2$?
b) What is the difference between $-5^3$ and $(-5)^3$?
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b) $-5^3 = -(5^3) = -125$, while $(-5)^3 = -125$ (no difference)
23. a) Show that $3^2 + 4^2 = 5^2$
b) Are these statements true or false? Give a reason for your answer each time:
i) $(-3)^2 + (-4)^2 = (-5)^2$
ii) $(-13)^2 = 12^2 + (-5)^2$
iii) $8^2 = -10^2 - 6^2$
c) Show your work to a partner. Do they find your explanation clear?
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b) i) $(-3)^2 + (-4)^2 = 9 + 16 = 25$, and $(-5)^2 = 25$ → True ✅
ii) $(-13)^2 = 169$, $12^2 = 144$, $(-5)^2 = 25$, $144 + 25 = 169$ → True ✅
iii) $8^2 = 64$, $-10^2 = -100$ (⚠️ actually equals $-100$ if negative outside the square), $6^2 = 36$, so RHS = $-100 - 36 = -136$ → False ❌
c) Answers may vary; students should explain each step clearly.
🧠 Think like a Mathematician
Question: How can we find two solutions to quadratic equations like $x^2 + x = n$? What patterns do you notice?
Equipment: Pencil, paper, calculator (optional)
Method:
- a) Here is an equation: $x^2 + x = 6$
- Show that $x = 2$ is a solution of the equation.
- Show that $x = -3$ is a solution of the equation.
- b) Here is another equation: $x^2 + x = 12$
- Show that $x = 3$ is a solution of the equation.
- Find a second solution to the equation.
- c) Find two solutions to this equation: $x^2 + x = 20$
- d) What patterns can you see in the answers to a, b and c?
Find some more equations like this and write down the solutions. - e) Compare your answers with a partner’s.
Follow-up Questions:
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- a.i:$2^2 + 2 = 4 + 2 = 6$ ✅
- a.ii:$(-3)^2 + (-3) = 9 - 3 = 6$ ✅
- b.i:$3^2 + 3 = 9 + 3 = 12$ ✅
- b.ii: Second solution is $x = -4$ because $(-4)^2 + (-4) = 16 - 4 = 12$
- c: Solutions are $x = 4$ and $x = -5$ (since $4^2 + 4 = 20$ and $(-5)^2 + (-5) = 25 - 5 = 20$)
- Pattern: The solutions are symmetrical: one positive, one negative, and they differ by 1 in absolute value.
❓ EXERCISES
24. a) Copy and complete this table.
| $x$ | $x - 1$ | $x^3 - 1$ | $x^2 + x + 1$ |
|---|---|---|---|
| 2 | 1 | 7 | |
| 3 | 2 | 13 | |
| 4 | |||
| 5 |
b) What pattern can you see in your answers?
c) Add another row to see if the pattern is still the same.
d) Add three rows where $x$ is a negative integer.
Is the pattern still the same if $x$ is a negative integer?
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| $x$ | $x - 1$ | $x^3 - 1$ | $x^2 + x + 1$ |
|---|---|---|---|
| 2 | 1 | 7 | 7 |
| 3 | 2 | 26 | 13 |
| 4 | 3 | 63 | 21 |
| 5 | 4 | 124 | 31 |
For $x = 2$: $x^2 + x + 1 = 2^2 + 2 + 1 = 4 + 2 + 1 = 7$ ✅
For $x = 3$: $x^3 - 1 = 27 - 1 = 26$ ✅
For $x = 4$: $x - 1 = 3$, $x^3 - 1 = 64 - 1 = 63$, $x^2 + x + 1 = 16 + 4 + 1 = 21$
For $x = 5$: $x - 1 = 4$, $x^3 - 1 = 125 - 1 = 124$, $x^2 + x + 1 = 25 + 5 + 1 = 31$
Pattern: It appears that $x - 1$, $x^3 - 1$, and $x^2 + x + 1$ follow regular patterns that grow with $x$. Try $x = -1, -2, -3$ to test for negatives and observe whether symmetry or pattern holds.
25. Any number that can be written as a fraction is a rational number.
Examples are: $7\frac{3}{4}, -12, \frac{18}{25}, 6, \frac{1}{15}, -2\frac{9}{10}$
Here is a list of six numbers:
$5,\quad -\frac{1}{5},\quad -500,\quad 16,\quad -4.8,\quad 99\frac{1}{2}$
🔎 Reasoning Tip
Rational numbers: Integers and fractions are both part of the set of rational numbers.
a) all the integers in the list
b) all the natural numbers in the list
c) all the rational numbers in the list
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b) Natural numbers: 5, 16
c) Rational numbers: all of them — $5,\ -\frac{1}{5},\ -500,\ 16,\ -4.8,\ 99\frac{1}{2}$
26. This Venn diagram shows the relationship between natural numbers and integers.
N stands for natural numbers and I for integers.
🔎 Reasoning Tip
Integers and rational numbers: Remember, all integers are part of the rational number set.

b) Write each of these numbers in the correct part of the diagram:
$1,\ -3,\ 7,\ -12,\ 41,\ -100,\ 2\frac{1}{2}$
c) Add another circle to your Venn diagram to show rational numbers.
d) Add these numbers to your Venn diagram:
$-8\frac{3}{7},\quad \frac{3}{5},\quad 0,\quad 6.3,\quad -\frac{10}{3}$
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- Integers (I but not N): –3, –12, –100
- Outside both (not integers): $2\frac{1}{2}$
d) - Rational numbers: all can be placed in the rational circle - Integers: 0
- Rational but not integer: $-8\frac{3}{7},\ \frac{3}{5},\ 6.3,\ -\frac{10}{3}$