this is an infinite series
its summation = initial value / (1-common ratio)
initial value when n=1 is (-1/3)^(1-1) = 1
common ratio is -1/3
summation = 1/(1-(-1/3))
= 1/(1+1/3)
=3/4
=0.75
sum = A/(1-R) = 1/[1-(-1/3)] = 1/(4/3)
= 3/4 = 0.75
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