Jawabanmu

2014-01-23T09:14:14+07:00
Sa+ar+ar2+…
Stakhingga=a/(1-r)  <=> 72 = a/(1-r)) maka  a = 72(1-r)… (1) a+ar^2+ar^4+…
 a = 48(1-r2)……(2)
1 dan 2   
  72(1-r) = 48(1-r)(1-r)
 r = ½   dan a = 36 U3  =ar^2= 36.1/4 = 9