A g.p has 8 terms.If the sum of first 4 terms is 45 and the sum of the last 4 term is 720, find the common ratio and the first term.

Answer 1

S4 = a (r^4-1)/(r-1) = 45
S8 = a(r^8-1)/(r-1) = 45+720 = 765

divide and you have (r^8-1)/(r^4-1) = 765/45 = 17
r^8-1 = (r^4-1)(r^4+1)
so, r^4+1 = 17
r^4 = 16
r = ±2
Now just find a

Answer 2

How did you get -+2

Answer 3

I didn't understand the 2nd line

Answer 4

Op