Powers & Roots (Edexcel IGCSE Maths A): Revision Note

Powers & Roots

What are powers (indices)?

• Powers (or indices) are the small 'floating' values that are used when a number is multiplied by itself repeatedly

  • 6¹ means 6

  • 6² means 6 × 6

  • 6³ means 6 × 6 × 6

• The big number at the bottom is called the base

• The small number that is raised is called the index, power, or exponent

• Any non-zero number to the power of 0 is equal to 1

  • 3⁰ = 1

• Any number to the power of 1 is equal to itself

  • 3¹=3

What are square roots?

• Roots are the reverse of powers

• A square root of 25 is a number that when squared equals 25

  • The two square roots of 25 are 5 and -5 

    • 5² = 25 and (-5)² = 25

• Every positive number has two square roots

  • One is positive and one is negative

  • Negative numbers do not have a square root

• The notation   refers to the positive square root of a number

  • 

  • You can show both roots at once using the plus or minus symbol ±

  • Square roots of 25 are 

What are cube roots?

• A cube root of 125 is a number that when cubed equals 125

  • The cube root of 125 is 5

    • 5³ = 125

  • Unlike square roots, each number only has one cube root

  • Every positive and negative number has a cube root

  • The notation refers to the cube root of a number

    • 

What are n^th roots?

• An nth root of a number is a value that when raised to the power n equals the original number

  • 3⁵=243 therefore 3 is a 5th root of 243 

• If n is even, there will be a positive and negative nth root

  • The 6th roots of 64 are 2 and -2

    • 2⁶ = 64 and (-2)⁶ = 64

  • The notation  refers to the positive nth root of a number

    • 

  • Negative numbers do not have an nth root if n is even

• If n is odd then there will only be one nth root

  • The 5th root of -32 is -2

    • (-2)⁵ = -32

  • Every positive and negative number will have an nth root

How do I estimate a root?

• You can estimate roots by finding the closest integer roots

  • To estimate

    • We know that  and 

    • So  must be between 4 and 5

What are reciprocals?

• The reciprocal of a number is the number that you multiply it by to get 1

  • The reciprocal of 2 is 

  • The reciprocal of  is 4

  • The reciprocal of  is 

• The reciprocal of a number can be written as an index of -1

  • 5^-1 is the reciprocal of 5, so 

• This can be extended to other negative indices

  • 5^-2 means the reciprocal of 5², so  or