Question

Tentukan jumlah 5 suku pertama dari deret geometri 8+4+2,...???

Answer 1

A = 8
r = 4/8 = 1/2

a= 8
u2 =ar = 8x1/2= 4
u3= ar^2 = 8 x 1/4= 2
u5 = ar^4 = 8 x 1/16 = 1/2

lima suku : 8,4,2,1, 1/2
Jumlah 5 suku :
S5 = a (1 -r^5) / (1-r)
     = 8 ( 1 -(1/32)) / ( 1 -(1/2)
     = 8 ( 32/32 - 1/32 ) / (1/2)
     = 8 ( 31/32) x 2
     = 16 (31/32)
     =  31/2
      = 15  1/2
     = 15,5

Answer 2

A = 8
r = 1/2

S5 = a(1 - r^5) / 1-r
= 8 (1-(1/2)^5) / 1 - 1/2
= 8(63/64) / 1/2
= 15,75