Answer:
(b) 65/27
Explanation:
Here, we notice the index for the summation runs from 0 to 3, so there are a total of 4 terms.
The sum represented by the summation symbol is ...
5(-2/3)^0 +5(-2/3)^1 +5(-2/3)^2 +5(-2/3)^3
= 5(1 -2/3 +4/9 -8/27)
= 5/27(27 -18 +12 -8)
= 5(13/27) = 65/27
__
There is a formula for the sum of a geometric series:
Sn = a1×(1 -r^n)/(1 -r)
Here, the first term is a1 = 5(-2/3)^0 = 5. The common ratio is the base of the exponential, r = -2/3. So, the sum of n=4 terms is ...
S4 = 5×(1 -(-2/3)^4)/(1 -(-2/3)) = 5×(1 -16/81)/(5/3) = 65/27