SOLUTION: t3 = 20, t6 = 160, s5 = ?. The third term is 20 and the sixth term is 160. What is the sum of the first 5 terms. This is part of geometric series questions so can you plz show me

Question 1108441: t3 = 20, t6 = 160, s5 = ?.
The third term is 20 and the sixth term is 160. What is the sum of the first 5 terms. This is part of geometric series questions so can you plz show me how to do it using formulas.
Thankyou
Answer by KMST(5328) (Show Source): You can put this solution on YOUR website!
Teachers like formulas, but the problem is that the same formula could be written differently by different teachers.
With some luck, the formulas your teacher uses formulas that look like the ones I will type below.

The first formula, applied for and says that

and

.
The problem gives us the value of those terms, so
and .
Dividing one by the other,

Going back to and substituting , we get

.
Now we can apply the second formula for ,

,
and substituting the values found before for and ,

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