Generated by claude-sonnet-4-6 · 2026-06-15 10:32 · grounding rag
Model Answer
Given AP: 30, 24, 18, …
Here, $a = 30$, $d = 24 - 30 = -6$, $n = 30$
Using the formula:
$$S_n = \frac{n}{2}[2a + (n-1)d]$$
$$S_{30} = \frac{30}{2}[2(30) + (30-1)(-6)]$$
$$= 15[60 + 29 \times (-6)]$$
$$= 15[60 - 174]$$
$$= 15 \times (-114) = \mathbf{-1710}$$
Source: Chapter 5, Exercise 5.3
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Explanation
- Examiners award 1 mark for correctly identifying $a$, $d$, $n$ and writing the formula, and 1 mark for the correct final answer.
- The key here is noting that $d$ is negative (−6), which makes the sum negative — a common error is using $d = +6$.
- Always substitute carefully and simplify step by step to avoid sign mistakes.