9514 1404 393
Answer:
71.447296
Step-by-step explanation:
The sequence is geometric with first term 100 and common ratio -0.4. Then the sum of n terms is ...
Sn = 100(1 -(-0.4)^n)/(1 -(-0.4))
and the sum of 9 terms is ...
S9 = 100(1 -(-0.4^9))/(1.4) = 71.447296
