the limiting sum of a geometric series is used when a series converges... so the formula is S∞=a1−r a = 1st term and r is the common ratio. the value of r will be |r|<1 this is what makes it converge In your question you will need to find r, the common ratio to allow you to find the 4 terms so using your information 10=21−r 1−r=15 which will result in r=45 so to find the 2nd, 3rd and 4th terms use the formula Tn=arn−1 you know the 1st term T2=2×45 T3=T2×45 T4=T3×45 hope this helps