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 can instead be written as or
- This means “the sum of all the terms from to for the sequence described by "
- would mean
- Using the following relations, summations can be grouped together (or ungrouped) to make some calculations easier:
- a and b are constants, and and are a functions of r
- Note that the top and bottom summation limits ( and ) 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- This is important – if the top and bottom limits don’t all match then the relation is no longer valid!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 or or , as and (or or ) are straightforward to find using formulae
- A useful result to remember is that
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 natural numbers is given by
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- This formula is not given in the formula book
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- The sum of the first square numbers is given by
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- This formula is given in the formula book
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- The sum of the first cube numbers is given by
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- This formula is given in the formula book
- Notice that this is equal to the formula for the sum of the first natural numbers, squared
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- 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,
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
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