# math

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. 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?

• math -

The cost is the arithmetic progression,
A(n)=A(1)+(n-1)d
where A(1)=100, d=25
and the sum to the nth term is
S(n)=(n/2)(A(1)+A(n)

For n=9 (90')
S(n)=(9/2)(100 + 100+(9-1)*25)
=?

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