Solution: (a? Answer? Answer? )/(a？ Answer? Answer? )=a？ /a？ =a？ q？ /a？ =q？ =24/3=8,∴q=2.

Answer? Answer? Answer? =(a？ )? q？ =3, so a? =(3/8)^( 1/3)=(? 3)/2

∴a？ Answer? Answer? =(a？ q？ )(a？ q？ )(a？ q？ )=(a？ )? (q？ )=(3/8)×2? =3×2? =3×64= 192.

2. If the sum of the first five terms of a geometric series is equal to 10 and the sum of the first ten terms is equal to 50, what is the sum of its first 15 terms?

Solution: S/S? =q？ +1=5, then q? =4

s？ =a？ (q？ - 1)/(q- 1)=a？ (4- 1)/[4^( 1/5)- 1]=3a？ /[4^( 1/5)- 1]= 10

So a? = 10[4^( 1/5)- 1]/3

∴s={( 10/3)[4^( 1/5)- 1](q- 1)}/(q- 1)={( 10/3)[4^( 1/5)- 1](64- 1)/[4^( 1/5)- 1]=( 10/3)×63=2 10