geometric series: sum_{k=1,n} 25(0.2)^{k-1} and sum_{k=1,n} 8(-1/2)^k

[senakuny]

I am working on question c. I got answer of 31.249 but the answer key says 31.245. What am I doing wrong?
Also when I'm using the geometric series equation, do I do 10-1 (n-1) exponent or keep it 10 (just n) exponent since question says k-1 exponent unlike question d that just says k exponent.

 

[blamocur]

I get 32.25 for c), and -2.664 for d). In c) you have wrong exponent: it should be [imath]\left(\frac{1}{5}\right)^{10}[/imath], not [imath]\left(\frac{1}{5}\right)^{10-1}[/imath], but this does not effect the first 3 decimal places. In d) you forgot to factor out get [imath]-\frac{1}{2}[/imath], so you got the answer multiplied by [imath]-2[/imath].