Find the sum of the first 7 terms of the following geometric

Find the sum of the first 7 terms of the following geometric series. 1/18 - 1/6 + 1/2 -... The sum is. (Type an integer or a simplified fraction.)

Solution

This series isn in the form of geometric series

where a₀=1/18    common ratio= -1/6/1/18 =-3

n=7

S_n=a₀(1-rⁿ)/(1-r)   = (1/18)(1-(-3)^7)/(1-(-3)) = (1/18)(2188)/4 = 547/18