Sums of Integers, Squares & Cubes (Edexcel A Level Further Maths: Core Pure)

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Jamie W

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Jamie W

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Sums of Integers, Squares & Cubes

How can we use sigma notation?

  • When writing the sum of a series you can use sigma notation
    • The series u subscript 1 plus u subscript 2 plus u subscript 3 plus... plus u subscript n minus 1 end subscript plus u subscript n can instead be written as sum from r equals 1 to r equals n of u subscript r or sum from r equals 1 to n of u subscript r
    • This means “the sum of all the terms from u subscript 1 to u subscript nfor the sequence described by u subscript r"
    • sum from r equals 1 to 3 of 2 r plus 3 would mean left parenthesis 2 left parenthesis 1 right parenthesis plus 3 right parenthesis plus left parenthesis 2 left parenthesis 2 right parenthesis plus 3 right parenthesis plus left parenthesis 2 left parenthesis 3 right parenthesis plus 3 right parenthesis equals 21
  • Using the following relations, summations can be grouped together (or ungrouped) to make some calculations easier:

sum from r equals p to q of a straight f left parenthesis r right parenthesis plus b space equals sum from r equals p to q of a straight f left parenthesis r right parenthesis plus sum from r equals p to q of b space equals space stack a sum with r equals p below and q on top straight f left parenthesis r right parenthesis plus sum from r equals p to q of b

sum from r equals p to q of a straight f left parenthesis r right parenthesis plus b straight g left parenthesis r right parenthesis space equals sum from r equals p to q of a straight f left parenthesis r right parenthesis plus sum from r equals p to q of b straight g left parenthesis r right parenthesis space equals space stack a sum with r equals p below and q on top straight f left parenthesis r right parenthesis plus b sum from r equals p to q of straight g left parenthesis r right parenthesis

  • a and b are constants, and straight f left parenthesis r right parenthesis and straight g left parenthesis r right parenthesis are a functions of r
  • Note that the top and bottom summation limits (p and q) are the same for all the sums
    • This is important – if the top and bottom limits don’t all match then the relation is no longer valid!
  • This can be very useful when straight f left parenthesis r right parenthesis or straight g left parenthesis r right parenthesis equals r comma space r squared or r cubed, as sum from blank to blank of b and sum from blank to blank of r (or r squared or r cubed) are straightforward to find using formulae
  • A useful result to remember is that sum from r equals 1 to n of a equals a plus a plus a plus... left parenthesis n space times right parenthesis equals n cross times a

What are the formulae for finding sums of integers, squares, and cubes?

  • There are several useful formulae for summing integers, square numbers, and cube numbers
  • The sum of the first n natural numbers is given by 
    • sum from r equals 1 to n of r equals 1 half n left parenthesis n plus 1 right parenthesis
      • This formula is not given in the formula book
  • The sum of the first n square numbers is given by
    • sum from r equals 1 to n of r squared equals 1 over 6 n left parenthesis n plus 1 right parenthesis left parenthesis 2 n plus 1 right parenthesis 
      • This formula is given in the formula book
  • The sum of the first n cube numbers is given by
    • sum from r equals 1 to n of r cubed equals 1 fourth n squared left parenthesis n plus 1 right parenthesis squared 
      • This formula is given in the formula book
      • Notice that this is equal to the formula for the sum of the first n natural numbers, squared
  • Using the relations given above, a more complicated summation can often be broken down into sums of constants, natural numbers, squares, and cubes
    • For example, sum from blank to blank of 2 r cubed minus 3 r squared minus 8 r minus 3 equals 2 sum from blank to blank of r cubed minus 3 sum from blank to blank of r squared minus stack 8 sum with blank below and blank on top r minus sum from blank to blank of 3

Examiner Tip

  • You can find summations using sigma notation on most advanced scientific calculators or graphics calculators – you can use this to check your answers
  • Bear in mind, however, that the question will normally require you to show your full working

Worked example

Find sum from r equals 1 to 7 of left parenthesis 2 r plus 1 right parenthesis left parenthesis r minus 3 right parenthesis left parenthesis r plus 1 right parenthesis

3-2-1-edx-a-fm-we1a-soltn

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Jamie W

Author: Jamie W

Expertise: Maths

Jamie graduated in 2014 from the University of Bristol with a degree in Electronic and Communications Engineering. He has worked as a teacher for 8 years, in secondary schools and in further education; teaching GCSE and A Level. He is passionate about helping students fulfil their potential through easy-to-use resources and high-quality questions and solutions.