9. How many poles will be in a pile of telephone poles if there are 30 in the bottom layer, 29 in the 2nd layer and so on until there in one in the top layer?
+36916 Arithmetic sequence d = 1
a1 = 30
a30 = 1
Sn= n(a1+a30)/2 = 30 (30+1)/2 = 465 poles
