Deret Geometri 10+5+2+1+... Tentukan jumlah 7 suku pertama? tolong dijawab

10+5+2+1+...
a=10
r=1/2
Sn=a(1 - r^n) : (1-r)
S7
=10(1-(1/2)^7) : (1-1/2)
= 10(1-1/128) : (1/2)
=10(128/128-1/128) : (1/2)
=10 x 127/128 x 2/1
= 2540/128
= 19,84