Question

Diketahui barisan geometri dengan suku ke-3=2 dan ke-7=32.suku ke 10 barisan tersebut adalah...

Answer 1

Un = ar^(n-1)
u3 = ar^2
u7 = ar^6

ar^6 / ar^2 = 32 / 2
r^4 = 16
r = 2

ar^2 = 2
a(2)^2 = 2
4a = 2
a = 2/4
a = 1/2

Sn = a(r^n - 1) / r-1
= 1/2(2^10 - 1) 2 - 1
= 1/2(1024 - 1)
= 1/2 (1023)
= 511,5

Answer 2

U-3 = 2   -> 2a+b = 2
U-7 = 32 -> 6a+b = 32
                ------------------ -
                 -4a    = -30
                   a     = 7,5

2a+b = 2
2(7,5)+b = 2
15+b     = 2
      b     = 2-15
      b     = -13

U10 = a+(n-1)b
      = 7,5 + (10-1) (-13)
      = 7,5-117
      = -109,5