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I'm not sure how to find the first two terms of a sequence (I got a few right, but most wrong and I don't know what's wrong). One of the problems is: a5 = 20; a8 = 4/25.

I set the problem up in this manner: 4/25 = -20r^3, but it does not turn out to be a perfect cube and the answer has no radicals in it, so it must work somehow.

Also: a1=2, a2=5, an=625/8, n=? I tried to find the common ratio, which I though was 2/5 (unless I forgot my basic algebra??), but it the whole problem is messed up.

Only one more thing, I promise! I'm pretty much altogether confused on geometric series. I was given the formula:

Sn= (a1-a1r^n)/1-r where r= common ratio and n is the number of terms in the series. One of the problems was: find the sum of the series: a1= 3/5, r=2, n-7. When I did this, it turned out to be a horrible number, when in fact the answer was supposed to be 47 and 5/8. If anyone could please just explain some of these things to me (I was making up a quiz while the teacher was explaining it). Thank you so much!