Jumlah 8suku pertama deret geometri 1/2 + 3/2 + 9/2....adalah?

r = 3/2÷1/2 = 3
Sn= (a((r^n)-1))/(r-1)
    = (1/2((3^8)-1))/(3-1)
    = (1/2(6561-1))/(2)
    = 1640