I dont understand how too solve this, help please - what is the sum of the first five terms of a geometric series with a1=10 and r=1/5?

Question

mathmale · Answer

One way to answer this question would be to calculate the first five terms of this series and then to add them up.

Or, you could use a more sophisticated method:  the formula for the sum of the first n terms of a geometric sequence of initial value a and of common ratio r.

Which approach do your instructional materials mention and illustrate?

anonymous · Answer

i can actually use both

mathmale · Answer

Note that if your first term, a, is 10, and your common ratio, r, is 1/5, then the 2nd term is 

$$10(\frac{ 1 }{ 5 })=?$$

anonymous · Answer

expect right now im trying partial sum

anonymous · Answer

second tern would be 5

mathmale · Answer

Yes, and there is a formula for the nth partial sum.  What is that formula?

Don't agree with the second term's being 5.  What is (10)(1/5)?

anonymous · Answer

well if you multiply 10(1/5) it will be 10/5 ohhh 2

mathmale · Answer

Right, and what is the formula for the nth partial sum of a geom series?

anonymous · Answer

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