Question 4 - A-Level Maths

Exam Board	AQA
Module	Further Paper 2 (Further Paper 2)
Year	2019
Session	June
Topic	Arithmetic Sequences and Series

4 The positive integer $k$ is such that $$\sum _ { r = 1 } ^ { k } ( 3 r - k ) = 90$$ Find the value of $k$.