Find the sum

1 + (–2) + (–5) + (–8) + …+ (–236)


Here First term, a = 1


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


Last term, an = - 236


As we know


an = 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


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