What are the first five partial sums of the geometric series: an = (5/8)*3^n-1

Student Answer: 0.625, 1.875, 5.625, 16.875, 50.625
1.875, 3.75, 7.5, 13.1250, 20.625
2.5, 4.375, 10, 26.875, 77.5
0.625, 2.5, 8.125, 25, 75.625

Question

What are the first five partial sums of the geometric series: an = (5/8)*3^n-1

 	Student Answer:	 0.625, 1.875, 5.625, 16.875, 50.625
 		 1.875, 3.75, 7.5, 13.1250, 20.625
 		 2.5, 4.375, 10, 26.875, 77.5
 		 0.625, 2.5, 8.125, 25, 75.625

anonymous · Answer

ugh i got to show work

anonymous · Answer

i am not good at this stuff

amistre64 · Answer

ab^0 + ab^1 + ab^2 + ab^3 + ab^4

a (1 + b + b^2 + b^3 + b^4)

seems doable, theres prolly even a nice little formula for it

amistre64 · Answer

$$a\frac{1-b^{n+1}}{1-b}$$
maybe

amistre64 · Answer

the +1 i think is a bad recollection :)  -b^n  should do it

anonymous · Answer

i am confused lol