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Point A and B of a triangle ABC are at (-1,1) and (3,9), respectively; while point C is on the parabola y=x2. Find coordinate of C so that the area of ABC is minimum, and calculate the largest are of ABC.

  • math -

    Let AB be the base, which will be a fixed length
    AB = √(4^2 + 8^2) = √80

    The height from C to line AB must be as long as possible, this is achieved when the slope of the tangent at C is the same as the slope of AB,

    dy/dx = 2x
    slope of AB = 8/4 = 2

    then 2x = 2
    x = 1

    when x = 1, y = 1^2 = 1

    C must be (1,1)

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