Square Root & Cube Root Symbols for Solving Equations - 8th Grade Math

Section 1

Where Did Root Symbols Come From?

People have been solving equations like x² = 9 for thousands of years. Long before anyone wrote the √ symbol, ancient mathematicians figured out how to find the "side of a square" when they knew its area. Let's look at how this idea developed over time.

~1800 BCE

Babylonian clay tablets show that ancient scribes could calculate square roots using step-by-step methods. They didn't have symbols — they described the process in words and tables.

~300 BCE

Euclid's Elements described how to construct a length that represented a square root using only a compass and straightedge. The Greeks connected square roots directly to geometry — the root was literally a side of a square.

1525 CE

Christoff Rudolff, a German mathematician, introduced the radical symbol √ in his algebra textbook. The symbol likely evolved from a stylized letter "r" (for the Latin word radix, meaning "root").

1637 CE

René Descartes refined the notation by adding the horizontal bar (called a vinculum) over the number inside the root: √‾. He also helped popularize writing exponents as superscripts, like x².

Today

We use √ for square roots and ∛ (with a small 3) for cube roots every day in math class, science, and engineering. These symbols give us a clean, exact way to write solutions.

So here's the big question this lesson answers: when you have an equation like x² = 5 or x³ = 10, how do you write the exact answer — not a long decimal, but a neat symbol that tells everyone precisely what the solution is?

Section 2

Core Definitions You Need to Know

Before we start solving equations, let's nail down four key ideas. Each one builds on the last, so take your time with them.

Perfect Squares & Perfect Cubes

A perfect square is what you get when you multiply a whole number by itself: 1, 4, 9, 16, 25, … A perfect cube is what you get when you multiply a number by itself three times: 1, 8, 27, 64, 125, …

Square Root (√)

The square root of p, written √p, is the positive number that, when multiplied by itself, gives you p. For example, √25 = 5 because 5 × 5 = 25.

Cube Root (∛)

The cube root of p, written ∛p, is the number that, when multiplied by itself three times, gives you p. For example, ∛8 = 2 because 2 × 2 × 2 = 8.

Positive Rational Numbers

A positive rational number is any positive number you can write as a fraction (like ¼, 3, or 7/2). In our equations x² = p and x³ = p, the value p is always a positive rational number.

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Key Takeaway

Think of a square root like this: if you have a square-shaped garden with an area of 49 square feet, the side length is √49 = 7 feet. The root "undoes" the squaring. A cube root works the same way but with a cube — if a cube-shaped box has a volume of 64 cubic inches, each edge is ∛64 = 4 inches.

Section 3

Seeing Roots in Action

Let's make this visual. The diagram below shows the connection between squaring a number and taking a square root. Notice how one operation "undoes" the other — they're inverse operations.

Diagram showing the relationship between squaring and taking a square root, and cubing and taking a cube root.

Look at the diagram above. On the left, you start with a side length x, square it to get the area p, and then use the square root symbol √ to go back to x. On the right, the same idea works in three dimensions: an edge length x cubed gives volume p, and the cube root ∛ takes you back. The root symbol is your "undo button" for exponents.

Section 4

The Equations & How to Solve Them

Now let's be precise about the math. You'll work with two types of equations in this lesson. Each one has a simple pattern for finding the answer.

Equation Type 1 — Square Root

x² = p → x = √p

"If x squared equals p, then x equals the square root of p."

Here's what's happening: since x² means "x times x," the square root asks, "What number times itself gives me p?" When p is positive, the principal (positive) square root is written √p. Notice that −√p also works as a solution because a negative times a negative is positive. So technically, x² = p has two solutions: x = √p and x = −√p. We often write this as x = ±√p.

Equation Type 2 — Cube Root

x³ = p → x = ∛p

"If x cubed equals p, then x equals the cube root of p."

Cube roots are a bit different. Since x³ means "x times x times x," the cube root asks, "What number, used three times in a multiplication, gives me p?" When p is positive, there is only one real solution: x = ∛p. There's no ± here because a negative cubed gives a negative result, not a positive one.

Important Distinction

x² = p → x = ±√p (two answers) x³ = p → x = ∛p (one answer)

Square-root equations give two solutions; cube-root equations give one.

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Key Takeaway

Think of it like a combination lock. If you know the "result" (the area or volume), the root symbol is the key that unlocks the original number. For squares, you get two keys (positive and negative). For cubes, you get exactly one.

Section 5

Perfect vs. Non-Perfect Values

Sometimes the number under the root sign comes out to a nice whole number. Other times it doesn't — and that's completely okay! The root symbol itself is the exact answer. Let's break this down.

Number line showing perfect squares and non-perfect squares, with their square roots labeled.

Look at the number line above. The blue dots sit exactly on whole numbers — those are perfect squares like √4 = 2 and √9 = 3. The colored dots fall between whole numbers. For example, √2 is between 1 and 2. Its decimal goes on forever (1.41421356…), so the cleanest way to write the exact answer is just √2.

Equation	Exact Solution	Decimal Approximation	Type
`x² = 36`	`x = ±6`	`±6.000`	Perfect square
`x² = 10`	`x = ±√10`	`±3.162…`	Non-perfect
`x² = ¼`	`x = ±½`	`±0.500`	Perfect square (fraction)
`x³ = 125`	`x = 5`	`5.000`	Perfect cube
`x³ = 20`	`x = ∛20`	`2.714…`	Non-perfect
`x³ = 8/27`	`x = ⅔`	`0.667…`	Perfect cube (fraction)

The table above shows the pattern. When p is a perfect square or perfect cube, the root simplifies to a nice number. When it isn't, the root symbol itself is the exact answer. That's exactly why we have the √ and ∛ symbols — they let us write precise solutions without rounding.

Section 6

Worked Example — Step by Step

Let's solve two equations from start to finish. Follow each step carefully.

Problem A: Solve x² = 50

1
Step 1 — Identify the equation type

The variable x is being squared (raised to the 2nd power). So we need a square root to solve it.
2
Step 2 — Apply the square root to both sides

Take the square root of both sides of the equation: √(x²) = √50 The left side simplifies because the square root "undoes" the square: x = ±√50 We write ± because both a positive and a negative number, when squared, give 50.
3
Step 3 — Simplify if possible

Is 50 a perfect square? Let's check: 7² = 49 and 8² = 64. Since 50 is between them, it's not a perfect square. But we can simplify: 50 = 25 × 2, and 25 is a perfect square!

√50 = √(25 × 2) = √25 × √2 = 5√2
4
Step 4 — Write the final answer

In decimal form, that's approximately ±7.071, but ±5√2 is the exact answer.

x = ±5√2

Problem B: Solve x³ = 40

1
Step 1 — Identify the equation type

The variable x is being cubed (raised to the 3rd power). So we need a cube root.
2
Step 2 — Apply the cube root to both sides

Take the cube root of both sides: ∛(x³) = ∛40 The left side simplifies: x = ∛40 No ± needed here — cubing and cube-rooting give just one answer for positive numbers.
3
Step 3 — Simplify if possible

Is 40 a perfect cube? 3³ = 27 and 4³ = 64. Since 40 is between them, it isn't a perfect cube. We can factor: 40 = 8 × 5, and 8 is a perfect cube!

∛40 = ∛(8 × 5) = ∛8 × ∛5 = 2∛5
4
Step 4 — Write the final answer

As a decimal, that's about 3.420, but 2∛5 is the exact answer.

x = 2∛5

Section 7

Square Roots vs. Cube Roots — Key Differences

These two types of roots are similar in many ways, but there are important differences you need to know. The table below lays them out side by side.

Feature	Square Root (√)	Cube Root (∛)
Equation form	`x² = p`	`x³ = p`
Undoes what?	Squaring (exponent 2)	Cubing (exponent 3)
Number of solutions	Two: x = +√p and x = −√p	One: x = ∛p
Why?	Both (+)² and (−)² are positive	Only (+)³ is positive
Geometric meaning	Side length of a square with area p	Edge length of a cube with volume p
Example (perfect)	`√49 = 7`	`∛64 = 4`
Example (non-perfect)	`√3 ≈ 1.732`	`∛10 ≈ 2.154`

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Key Takeaway

Here's an easy way to remember the difference in solutions: think about arguing with a friend. If you say "What number squared gives 9?" your friend could say +3 or −3, and both are right — that's why square-root equations have two answers. But if you say "What number cubed gives 27?" there's only one answer: 3. No argument! Cube-root equations have just one solution.

Section 8

Looking Ahead: Connections to Future Math

The skills you're building right now lay the foundation for several topics you'll see in high school and beyond. Here's a sneak peek at where roots show up next.

What You're Learning Now	Where It Goes Next
Solving x² = p using √	The Quadratic Formula — In Algebra 1 and 2, you'll solve more complex equations like ax² + bx + c = 0. The formula uses a square root right in the middle!
Solving x³ = p using ∛	Higher-degree equations — You'll eventually encounter fourth roots, fifth roots, and beyond. The pattern is the same: nth root undoes the nth power.
Knowing √2 is irrational	The Real Number System — You'll classify numbers as rational or irrational. Understanding that many roots are irrational is a key part of that classification.
Simplifying √50 = 5√2	Simplifying radical expressions — In Algebra, you'll add, subtract, multiply, and divide expressions with roots, just like you do with regular numbers.
Writing x = ±√p	Exponent rules — Roots can be written as fractional exponents: √p = p^1/2 and ∛p = p^1/3. This connection unlocks powerful simplification techniques.

The big idea is that root symbols aren't just one-off tools — they're part of a whole family of operations connected to exponents. Every time you learn a new exponent rule in the future, there's a matching root rule to go with it. What you're practicing now will make all of that feel familiar.

Section 9

Practice Problems

Try these five problems on your own before peeking at the answers. They start easy and get harder — challenge yourself!

Question 1: Problem 1 — Conceptual

In your own words, explain why the equation x² = 16 has two solutions, but the equation x³ = 16 has only one solution.

Show answer

When you square a number, both the positive and the negative version give a positive result. For example, (+4)² = 16 and (−4)² = 16. So x² = 16 has two solutions: x = 4 and x = −4. But when you cube a number, only a positive number gives a positive result. (+2.52)³ is positive, but (−2.52)³ is negative. So x³ = 16 has just one (positive) solution: x = ∛16. Nice work if you noticed this difference!

Question 2: Problem 2 — Basic

Solve the equation x² = 81. Write the exact solution(s).

Show answer

Take the square root of both sides: x = ±√81. Since 81 is a perfect square (9 × 9 = 81), this simplifies to x = ±9. That means x = 9 or x = −9. You can check: 9² = 81 ✓ and (−9)² = 81 ✓.

Question 3: Problem 3 — Intermediate

Solve the equation x³ = ¹⁄₈. Express your answer as a fraction.

Show answer

Take the cube root of both sides: x = ∛(1/8). You can take the cube root of the numerator and denominator separately: ∛1 = 1 and ∛8 = 2. So x = ½. Check: (½)³ = ½ × ½ × ½ = ⅛ ✓. Great job if you remembered to apply the cube root to both the top and bottom of the fraction!

Question 4: Problem 4 — Applied

A square garden has an area of 45 square meters. What is the exact length of one side of the garden? Then estimate this length to the nearest tenth of a meter. (Hint: 6² = 36 and 7² = 49.)

Show answer

Since the area of a square is side², we need to solve s² = 45. The exact side length is s = √45. We can simplify: 45 = 9 × 5, and √9 = 3, so √45 = 3√5. To estimate: since 6² = 36 and 7² = 49, the answer is between 6 and 7. Using a calculator, √45 ≈ 6.7 meters. The exact answer, 3√5 meters, is what you'd write on a math test. The decimal is useful when you're actually measuring the garden!

Question 5: Problem 5 — Challenge

Solve x² = ⁷⁄₄. Write your answer using a square root symbol in simplified form. Then explain: is your answer rational or irrational?

Show answer

Take the square root of both sides: x = ±√(7/4). You can split this into: x = ±(√7 / √4) = ±(√7 / 2), which we write as x = ±√7/2. Is this rational or irrational? The number 7 is not a perfect square, so √7 is irrational (its decimal goes on forever without repeating). Dividing an irrational number by 2 still gives an irrational number. So x = ±√7/2 is irrational. This is a great stretch problem because it combines fraction skills, root simplification, and number classification all in one. Well done if you got it!

Summary

Lesson Summary

Summary

In this lesson, you learned that the square root symbol (√) and the cube root symbol (∛) are used to represent exact solutions to equations. When you face an equation of the form x² = p, you solve it by writing x = ±√p, which gives you two solutions (one positive and one negative). When you face x³ = p, you write x = ∛p, which gives you one solution.

If p happens to be a perfect square (like 4, 9, 25, or ¼) or a perfect cube (like 8, 27, 64, or ⅛), the root simplifies to a nice rational number. If not, the root symbol itself is the exact answer — decimals are only approximations. You also practiced simplifying roots by factoring (for example, √50 = 5√2) and saw that these skills connect directly to the quadratic formula, fractional exponents, and the broader real number system you'll explore in future courses.