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Math Help - Inversely proportional!?!

  1. #1
    Senior Member Mukilab's Avatar
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    Inversely proportional!?!

    Q is inversely proportional to the cube of T

    When T=5, Q=12.6

    5^3=125

    It's as if someone added a decimal point and then added a 0.1 although it is never so with maths....thankfully.
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  2. #2
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    Hello, Mukilab!

    Do you really understand proportionality?


    Q is inversely proportional to the cube of T.
    Q \:=\:\frac{k}{T^3} .[1]


    When T=5,\;Q=12.6
    Substitute into [1]: . 12.6 \:=\:\frac{k}{5^3} \quad\Rightarrow\quad k \:=\:1575

    Therefore: . Q \:=\:\frac{1575}{T^3}

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  3. #3
    Senior Member Mukilab's Avatar
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    Quote Originally Posted by Soroban View Post
    Hello, Mukilab!

    Do you really understand proportionality?


    Q \:=\:\frac{k}{T^3} .[1]


    Substitute into [1]: . 12.6 \:=\:\frac{k}{5^3} \quad\Rightarrow\quad k \:=\:1575

    Therefore: . Q \:=\:\frac{1575}{T^3}

    No I did not understand proportionality but now I do understand, thank you.
    If I only have T, let's say it is 3. How would I calculate Q. Surely I need this k (where did you get it from?)
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  4. #4
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    Proportionality afaik requires a variable equal to another variable multiplied (or divided) by a constant.

    X=kV ; X is proportional to V. An inverse proportionality would be X=\frac{k}{V}

    So the constant k either has to be given, or you need to be able to calculate it from the information you have.
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  5. #5
    Senior Member Mukilab's Avatar
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    Quote Originally Posted by dkaksl View Post
    Proportionality afaik requires a variable equal to another variable multiplied (or divided) by a constant.

    X=kV ; X is proportional to V. An inverse proportionality would be X=\frac{k}{V}

    So the constant k either has to be given, or you need to be able to calculate it from the information you have.
    The information given is: T is now 3. Work out Q. GCSE paper :/ don't think they made a mistake
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  6. #6
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    They did mention it was inversely proportional, right? That means;

    Quote Originally Posted by Soroban
    Q=\frac{k}{T^3}
    even though k isn't written anywhere you know there is a constant to define the relationship between Q and T. You can also call that constant whatever you like. It's basically rule 1 of algebra.
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