- Messages
- 46
- Reaction score
- 5
- Points
- 8
Thank you!You have to use the sum formula
Sn=n/2 (2a+(n-1)d)
in this case n is m
m/2 (2x161+(m-1)x-7)=0
322-7m+7=0
7m=329 m=47
(b) in geometric progression the sum of n terms formula is
a(1-r^n)/(1-r) and sum to infinity formula is a/1-r
s0 this becomes
a(1-r^n)/(1-r)<0.9 a/1-r as sum of n terms <90% of sum to infinity