Question

Suatu barisan geometri,U4 = 40 dan U6 = 160,tentukan S8!

Answer 1

Un=ar^n-1
U4=ar^4-1=40
U6=ar^6-1=160

ar^5/ar^3 = 160/40
r^2 = 4
r=√4 = 2
jadi, ar^3=40
a×2^3=40
8a=40
a=40/8=5
jadi, Sn=a(r^n - 1)/r-1
S8=5(2^8 - 1)/2-1
=5(256-1)/1
=1275