Question

find the sum of sequence ‐8, ‐5,‐2,....7
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Answer 1

here, first term(a) = -8
Last term(tn) = 7
Common difference (d) = -5-(-8) =-5+8 =3

We know,
tn = a+ (n-1)d
7 = -8 + (n-1)3
15 = (n-1)3
n-1 = 15/3 =5

n=5+1
n=6

Therefore, Sum of the series(Sn) = n/2{2a +(n-1)d}

= 6/2 {-8 *2 + (6-1)*3}
= 3 *{-16+15}
=3*-1
= -3