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Math Help - Directly proportional

  1. #1
    Senior Member Mukilab's Avatar
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    Directly proportional

    D is directly proportional to the square of t

    d=80 when t=4

    so

    d=kt^2

    k being the constant

    k=80/64

    Work out the positive value of t when d=45

    Can this be equated accurately without a calculator?
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  2. #2
    MHF Contributor harish21's Avatar
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    Quote Originally Posted by Mukilab View Post
    D is directly proportional to the square of t

    d=80 when t=4

    so

    d=kt^2

    k being the constant

    k=80/64 WRONG!! 4^2 = 16, NOT 64

    Work out the positive value of t when d=45

    Can this be equated accurately without a calculator?

    D is directly proportional to the square of t does mean:

    d=k . t^{2}

    d=80 when t=4 gives you:

    80 = k \times (4)^2 \implies k = \frac{80}{16}=5

    Now can you find t when d=45?
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