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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.Dictionary Definition
The Merriam-Webster dictionary defines a square root as "a factor of a number that when squared gives the 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:
√36 × √36 = 36
The square root of √36 is equal to 6. The square root of 36 (6), when multiplied by itself, gives 36:
6 × 6 = 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 1^{2} ("1 squared").
- 4 is a square number. It is 2 × 2, or 2^{2} ("2 squared").
- 9 is a square number. It is 3 × 3, or 3^{2} ("3 squared").
- 1 = 1^{2} ∴ √1 = 1.
- 4 = 2^{2} ∴ √4 = 2.
- 9 = 3^{2} ∴ √9 = 3.
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:
√2 × √2 = 2
√2 × √3 = √(2 × 3) = √6
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:
√x × √x = x
√x^{2} = x
√x × √y = √(xy)
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:
1² = 1 so √1 = 1
2² = 4 so √4 = 2
3² = 9 so √9 = 3
4² = 16 so √16 = 4
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.
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