Question

Find the sum of the first 80 terms of the sequence. Show all work for full credit. 1, -3, -7, -11, ...

Answer 1

-12960

Explanation:

The formula for the sum of terms =

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

n = number of terms = 80

The given sequence =

1, -3, -7, -11, ...

a = First term = 1

d = common difference

= Second term - First term

= -3 - 1

= -4

Therefore, the sum of the first 80 terms:

S80 = 80/2[2 × 1 + (80 - 1)]-4

= 40[2 + 79]-4

= 40 × 81 × -4

= -12960

The sum of the first 80 terms = -12960