diketahui deret geometri dengan suku ke-4 = 3, dan suku ke-7 = 81. jumlah 8 suku pertama adalah...

Deret Geomteri
Un = ar^(n-1)

U4 = ar^(4-1) = ar^3 = 3
U7 = ar^(7-1) = ar^6 = 81

ar^6 = 81
ar^3 . r^3 = 81
3 . r^3 = 81
r^3 = 81 : 3
r^3 = 27
r = ^3 dari 27 = 3

ar^6 = 81
a. 3^6 = 81
a . 729 = 81
a = 81 / 729
= 1 /9

Sn= a (r^n -1) / (r-1)
S8 = 1/9 (3^8 - 1 ) / (3-1)
= 1/9 (6561 -1)/ 2
= 1/9 (6560) /2
= 1/ 9 x 3280
= 3280 / 9
= 364 4/9