Find the sum

1 + (–2) + (–5) + (–8) + …+ (–236)

Here First term, a = 1

Common difference, d = - 2 - 1 = - 3

Last term, a_n = - 236

As we know

a_n = a + (n - 1)d

- 236 = 1 + (n - 1) (- 3)

- 236 - 1 = (n - 1) (- 3)

- 237 = (n - 1) (- 3)

n - 1 = 79

n = 80
Using the sum of first n terms formula, if last term is given

i.e,

= 80/2(1 - 236)

= 40 (- 235)

= - 9400