Answer:
-12960
Step-by-step 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
