Sequences

3 marks · Normal

Find the sum of the first 25 terms of the arithmetic sequence: 7, 12, 17, 22, ...

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1925 (±0)

Common mistakes: Forgetting (n-1) times d; Using wrong formula

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Marking points

M1 · 1 mark · Identify a=7, d=5, n=25 and state or use correct sum formula.

M2 · 1 mark · Find the 25th term correctly. (follow-through)

A1 · 1 mark · Calculate correct sum: 1925. (follow-through)

Worked solution

Here a = 7, d = 5, n = 25

Alternatively, use S_n = n/2 [2a + (n-1)d]