TMUA Practice · Paper 1 · Question 4

The sequence $a_{n}$ is defined by the rule:

$a_{n} = (- 1)^{n} - (- 1)^{n - 1} + (- 1)^{n + 2}$ for $n \geq 1$ .

Find the value of

n = 1 \sum 39 a_{n}