Square Roots
(KS2, Year 6)

The Lesson

The square root of a number, when multiplied by itself, gives that number. A square root is denoted by writing a √ (radical) symbol in front of a number.

A Real Example of a Square Root

The square root of 36 is written √36. The square root of 36 (√36), when multiplied by itself, gives 36: The square root of √36 is equal to 6. The square root of 36 (6), when multiplied by itself, gives 36:

The Square Root Is the Opposite of Squaring a Number

Finding a square root is the inverse (opposite) of squaring a number. (Don't forget: Squaring a number means multiplying the number by itself). Taking the square root of 36 gives √36. Squaring √36 gives 36.Remember that √36 is 6:

How to Find the Square Root of a Number

It can be difficult to find the square root of a number by hand. (It is easy on a calculator. Just press the √ button!) A different approach is suggested depending whether you are finding the square root of a square number or not.

Square Roots of Square Numbers

A square number a whole number (an integer) that results from a smaller whole number being multiplied by itself:

• 1 is a square number. It is 1 × 1, or 1² ("1 squared").
• 4 is a square number. It is 2 × 2, or 2² ("2 squared").
• 9 is a square number. It is 3 × 3, or 3² ("3 squared").

This means that if we square root a square number, we get a whole number:

• 1 = 1² ∴ √1 = 1.
• 4 = 2² ∴ √4 = 2.
• 9 = 3² ∴ √9 = 3.

The square roots of the first 10 square numbers are shown below:

Square Roots If It Is Not a Square Number

If number is not a square number, its square root will not be a whole number. For example, √2 = 1.414213562... Because √2 cannot be simplified to a whole number (or even a fraction), it is called a surd. It can be neater and more precise just to leave surds as they are: just write √2 as √2, not 1.414213562... A surd is an irrational number (it cannot be expressed as a fraction). As a decimal, it goes on forever. Surds have their own rules, for example:

Square Roots in Algebra

In algebra, letters are used instead of numbers. Just as we can find the square root of a number, such as √2... ...we can find the square root of a letter, such as √x. We would call √x the square root of x. Square roots of letters have their own rules, for example:

Why "Square" Roots?

A square root is the inverse (opposite) of a square number. If a number represented the area of a square, then the square root represents the length of the side of that square. A square with an area of 1 has sides of length 1. A square with an area of 4 has sides of length 2. A square with an area of 9 has sides of length 3.

There Are Always Two Square Roots

There are always two square roots for each number, a positive root and a negative root. The square roots of 36 are 6 and −6. It is conventional just to consider the positive square root, so we would say √36 is 6.

'Guess-timating' Square Roots As a Decimal

It is very difficult to work out a square root as a decimal without a calculator. But, it is possible to have a good guess from the ones we do know. The ones we know are the square roots of the square numbers: What is √5? 5 is between 4 and 9. We can therefore say √5 must be between √4 and √9, or between 2 and 3.