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Math Help - formulas/radius

  1. #1
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    formulas/radius

    a cylindrical concrete sewer pipe is to be cast so that the concrete is uniformly 75mm thick while the cross section of the concrete is 0.164m2(thats sposed 2 be squared)
    formulate and solve the simultaneous equations showing the relationship betwen the external radius, R, andthe internal radius r, of the pipe.

    all workin out would be great thnx
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  2. #2
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    Hello, jaswinder!

    A cylindrical concrete sewer pipe is to be cast so that the concrete
    is uniformly 75mm thick, while the cross section of the concrete is 0.164 m.
    Formulate and solve the simultaneous equations showing the relationship
    between the external radius, R, and the internal radius, r, of the pipe.

    They're mixing millimeters and meters . . . beware!
    . . 0.164 m = 164,000 mm

    The difference of radii is 75: .R - r .= .75 . . r .= .R - 75 .[1]

    The cross-sectional area is: .πR - πr .= .164,000 .[2]


    Substitute [1] into [2]: .πR - (R - 75) .= .164,000

    . . and we have: .150πR .= .5625π + 164,000

    . . . . . . . . . . . . .5625p + 164,000
    Therefore: . R .= .-------------------- . .385.5 mm
    . . . . . . . . . . . . . . . . 150p

    . . . . and: . r .= .310.5 mm

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