Question 1127671
For there to be a common ratio,

{{{(X+8)/(X+3) = (X+3)/(X+1)}}}

And after cross multiplying -

X^2+9X+8= X^2+6X+9


9X+8=6X+9

3X=1

i)  X= 1/3 

ii)  Therefore the first 3 numbers are  4/3, 10/3, 25/3 with a common ratio of 2.5

iii) = A (1+r^n)/(1-r) =  4/3 (1-2.5^15)/(1-2.5)


4/3 (-931321.575)/-1.5  =  827841.4


*[illustration sum001].


I used a spreadsheet to verify the solution is correct.