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Math Help - Year 10 GCSE Paper question varies inversely as the square

  1. #1
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    Year 10 GCSE Paper question varies inversely as the square

    The shutter speed, S, of a camera varies inversely as the square of the aperture setting, f.

    When f= 8, S= 125

    a) find a formula for S in terms of f.



    b) Hence, or otherwise calculate the value of S when f= 4

    thanks for any time anybody spends on this question

    Rob
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  2. #2
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    If x varies inversely to y, then:
     x = \frac{k}{y}
    Where k is the constant value for any values of (x,y).

    If x varies inversely to the square of y, then:

     x = \frac{k}{y^2}

    Find the constant, and then replace in the equation:
     S = \frac{k}{f^2}

    Use this equation to find (b).
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