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Posted by on Thursday, November 18, 2010 at 8:43am.

A particle is moving along the parabola 4y = (x + 2)^2 in such a way that
its x-coordinate is increasing at a constant rate of 2 units per second. How
fast is the particle's distance to the point (-2, 0) changing at the moment
that the particle is at the point (2, 4)?

  • DIFF. CALCULUS - , Thursday, November 18, 2010 at 9:15am

    Let the point be at the position P(x,y)

    The parabola has vertex at (-2,0), so the question is how fast is P moving away from the vertex.

    Let the distance be D
    D^2 = (x+2)^2 + y^2
    = (x+2)^2 + (x+2)^4/16

    2D dD/dt = 2(x+2) dx/dt + (1/4)(x+2)^3 dx/dt

    dD/dt = [2(x+2) dx/dt + (1/4)(x+2)^3 dx/dt]/(2D)

    when x = 2
    D^2 = 16 + 256/16
    = 32
    D = √32

    dD/dt = [2(4)(2) + (1/4(64)(2)]/(2√32)
    = 24/√32 = 6/√2 = appr. 4.243 units per second

    check my arithmetic

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