Question

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

a.
11654
c.
11656
b.
11657
d.
11655

Answer 1

D. 11655

Explanation:

First find the 90th term:

a90=a1+(n−1)d

= -4+89(3)=263

Then find the sum:

Sn = n(a1 + an) / 2

S90 = 90 (-4 + 263) /2

S90 = 23,310/2

S90 = 11,655