The question goes:

The first, second and fourth terms of a convergent geometric progression are consecutive terms of an arithmetic progression. Prove that the common ratio of the geometric progression is (-1 + sq root 5)/2.

The first term of the geometric progression is positive. Show that the sum of the first 5 terms of this progression is greater than the nine tenths of the sum to infinity.

I tried but I couldnt get the first part. As for the second part, I don't really understand the 'nine tenths' part?

Thanks in advance if you could help!