posted by jazzmin .
. A person hired a firm to build a CB radio tower. The firm charges $100 for labor for
the first 10 feet. After that, the cost of the labor for each succeeding 10 feet is $25
more than the preceding 10 feet. That is, the next 10 feet will cost $125, the next
10 feet will cost $150, etc. How much will it cost to build a 90-foot tower?
The cost is the arithmetic progression,
where A(1)=100, d=25
and the sum to the nth term is
For n=9 (90')
S(n)=(9/2)(100 + 100+(9-1)*25)