In a geometric sequence of real numbers, the sum of the first 2 terms is 7, and the sum of the first 6 terms is 91. What is the sum of the first 4 terms?

7==F * [1 - R^2] / [1 - R],

91==F * [1 - R^6] / [1 - R], solve for F, R

Use division to get:

F ==2.56218- this is the first term

R==1.73205 - this is the common ratio==sqrt(3)

Sum of the first 4 terms==[2.56218 * [1 - sqrt(3)^4] / [1 - sqrt(3) ]==28