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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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