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Thread: calc help

  1. #1
    Oct 2007

    calc help

    The figure below shows a road running in the shape of a parabola from the bottom of a hill at A to point B. At B it changes to a line and continues on to C. The equation of the road is represented by R(x), also shown below.

    B is 1000 feet horrizontally from A and 100 feet higher. Since the road is smooth, R'(x) is continuous. What is the value of b?

    Multiple Choice:
    A---> 0.2
    B---> 0.02
    C---> 0.002
    D---> 0.0002
    E---> 0.00002

    i was thinking that the answer is (A). because (100/1000)*2 = 0.2.

    thanks for the help guys
    Attached Thumbnails Attached Thumbnails calc help-asdfasdf112222333333.jpg  
    Last edited by DINOCALC09; Oct 25th 2007 at 08:20 PM.
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  2. #2
    MHF Contributor
    Apr 2005
    Let us imagine the line BC is straight, even if it ios not in the drawing.

    So R(x) has two branches that are continuous at B.

    The parabolic branch AB is
    R(x) = ax^2
    At B, it is (1000,100), so,
    100 = a(1000)^2
    a = 100 /(1000)^2
    a = 10^(-4) = 0.0001
    R(x) = (0.0001)x^2

    The tangent line at B has the slopes:
    --R'(x) = 2ax for the parabola, and,
    --R'(x) = b for the straight line.
    Since R'(x) = R'(x) at B, thenn
    b = 2ax
    b = 2(0.0001)(1000)
    b = 0.2 ----------------------answer.
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