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A-LevelMathematicsSeries9709/12 · May/June 2020 · Q45 Marks

The nth term of an arithmetic progression is (1/2)(3n – 15). Find the value of n for which the sum of the first n terms is 84.

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

  1. [M1] 1st term is -6, 2nd term is -4.5 (M1 for using kth terms to find both a and d)
  2. [A1 A1] → a = -6, d = 1.5
  3. [M1] Sn = 84 → 3n² - 27n - 336 = 0
  4. [A1] Solution n = 16

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About this question

Topic
Series, A-Level Mathematics (syllabus 9709). Structured question, 5 marks.
Source
A-Level Mathematics, May/June 2020 Paper 1 Variant 2, question 4.

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