Question

Find the sum of -5+(-8)+(-11)................+(-250)

Answer 1

By the formula of Sn

Sn=n/2[2a+(n-1)d] = n/2[a + An]

First we have to find n by An formula

An=a+(n-1)d

-250=-5+(n-1)(-3)

>-255=(n-1)(-3)

>n-1=85

>n=86

Sn=n/2[a + An]

Sn=86/2[-5-250] = 43[-255]=10965