Question

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

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


Please select the best answer from the choices provided

a) A
b) B
c) C
d) D

Answer 1

d.

11655

Explanation:

Answer 2

Answer: Sum of first 90 terms is Option d. 11655

Explanation:

We have to find the sum of first 90 terms of the sequence -4,-1,2,5,8......

First we find the 90th term of the sequence

As we know the formula A(n)= A(1)+(n-1)d

Given from the sequence

A(1) = (-4)

n=90

d = 3

So A(n) = -4+(90-1)3

= (-4)+89×3

= 263

Now we know formula for the sum of any sequence is

Sum S(n) =
`(n)/(2)(A(1)+A(n))`

Now we know n= 90

A(1) = -4

A(n) =263

Now we put the value in the formula

S(n)=
`(90)/(2)(-4+263)`

= 45×259

= 11655