Arithmetic Series (OCR A Level Maths: Pure)

How do I find the sum of an arithmetic series?

• An arithmetic series is the sum of the terms of an arithmetic sequence

• The following formulae will let you find the sum of the first n terms of an arithmetic series:

• • a is the first term
  • d is the common difference
  • l is the last term

• You can use whichever formula is more convenient for a given question
• The a and the d in those formulae are exactly the same as the ones used with arithmetic sequences

 

How do I derive the arithmetic series formula?

• Learn this proof of the arithmetic series formula – you can be asked to give it on the exam:
  • Write the terms out once in order
  • Write the terms out again in reverse order
  • Add the two sums together
    • The terms will pair up to give the same sum 
    • There will be n of these terms
  • Divide by two as two of the sums have been added together