Question

Find the sum of the first 90 terms of the sequence -4,-1,2,5,8....

Answer 1

common difference =  3 

First term = a  = -4
Last term = 90th term 
                = a + 89d
                = -4 + 89(3)
                = -4 + 267
                = 263 
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We know, sum of n terms = [first term + last term] × $\frac{n}{2}$

Here,

S(90) = [ -4 + 263] × $\frac{90}{2}$
          = [ 259] × 45
          = 11655


sum of first 90 terms = 11655


i hope this will help you

(-:

Answer 2

answer is 11655 I have explained it in that pic