The sum to infinity of a geometric progression is 5 and the sum to infinity of another series formed by taking the first, fourth, seventh and tenth,.....terms ( that is U1 + U4 + U7 +U10 +......) is 4. Find the common ratio of the first series. Answer: r = (-1+sq Root 2) / 2 Why is r = (-1-sq root 2)/2 rejected as an answer!