Cambridge AS & A Level. 2025. Mathematics (9709). May/June - Paper 1 (12) · Pure Mathematics 1 - Question 20

**(a)** The first, second and third terms of an arithmetic progression are $4k$, $k^2$ and $8k$ respectively, where $k$ is a non-zero constant.

**(ii)** Find the sum of the first 20 terms of the progression.

(In part (i) you found $k = 6$.)