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Math Help - quadratic help

  1. #1
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    fratta terme forli Italy
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    quadratic help

    hello if i having this problem
    a closed cylinder 15 cm high has a surface area of 1500 cm squared, find the radius of the cylinder.
    To solve this equation could i use the quadratic formula ? or is there another way I can do.

    Thank you for any helps
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  2. #2
    MHF Contributor
    Joined
    Apr 2005
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    1,631
    See if your final equation is quadratic in the form
    Ax^2 +Bx +C = 0
    Then, yes, you can use the Quadratic formula to find x.

    Closed right circular cylinder 15cm high, 1500sq.cm total surface area, and radius r.
    Find r.

    1500 = (2pi*r)(15) + 2(pi*r^2)
    Umm, yes, you can use the Quadratic formula here.

    1500 = 30pi*r +2pi*r^2
    2pi*r^2 +30pi*r -1500 = 0
    r = {-30pi +,-sqrt[(30pi)^2 -4(2pi)(-1500)]} / 2(2pi)
    r = {-94.2478 +,-215.8281} /12.5664
    r = 9.675cm or -24.675cm

    Reject the r = -24.675cm because there are no negative dimensions.

    Therefore, r = 9.675cm ---------------answer.
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