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Math Help - circular problem

  1. #1
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    circular problem

    I need help with this.... Can anyone help me??? Many thanks

    The figure shows a circular field with centre O, and radius 2 km. AC is the diameter of the circle, and B lies on the circumference. Jack cycles along the straight line from A to B at a speed of 12 km/hr. He then walks along the straight line from Bto C at a speed of 5 km/hr. Given that the distance from B to C is x km, and the total time taken is T hours, find the value of x so that Jack can complete his journey in the shortest time possible.
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  2. #2
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    a triangle inscribed in a semicircle is a right triangle.

    AC = 4 km

    BC = x km

    AB = \sqrt{16 - x^2} km

    distance/speed = time

    T = \frac{\sqrt{16 - x^2}}{12} + \frac{x}{5}

    find \frac{dT}{dx} and minimize ... don't forget about endpoint extrema.
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  3. #3
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    what is endpoint extrema?

    Hi skeeter, thanks for your answer. But erm can you teach me what is endpoint extrema? My teacher didn't teach me about it and this is the first time I come across this term. I know that I'm a bit of a noob but I hope you can teach me as I want to know more. And thanks again!!!!
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  4. #4
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    what is the domain of the function T(x) ?

    have you found \frac{dT}{dx} and completed an analysis of the function T(x) ?
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  5. #5
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    Re: Circular problem

    I work out the problem. Can the answer be 3.69 km?
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  6. #6
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    is x = 3.69 the location of a minimum or a maximum?

    how do you tell?
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  7. #7
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    Domain of T= -1.538 to 1.538

    dT/dx = -12x / [144 (√16-x)] + 1/5

    and 3.69 is a maximum.

    Am I correct? If not, can you correct my mistakes? Thanks!
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  8. #8
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    the domain for x is 0 \leq x \leq 4

    T(3.69) is a maximum ... the function T(x) increases and then decreases because T'(x) changes sign from (+) to (-).

    This means that the minimum for T(x) is either T(0) or T(4) ... endpoint minimums. the absolute min is the lesser of the two.
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