Answer:
5
Explanation:
We are given the geometric series:
-1, - 2, - 4, - 8.......
In the geometric series above, the common ratio = second term/first term = -2/-1
= 2
Where common ratio is greater than 1(r >1)
The sum of a geometric progression =
Sn = a(rⁿ -1)/r - 1
In the question we are asked to find the number of term = n
Sn = -31
a = -1
r = 2
Hence,
-31 = -1(2ⁿ - 1)/2 - 1
-31 = -1(2ⁿ - 1)
-31 = -2ⁿ + 1
2ⁿ = 1 + 31
2ⁿ = 32
2ⁿ = 2^5
n = 5
Therefore, the number of terms(n) = 5