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Homework Help: Related Rates

  1. Feb 15, 2008 #1
    1. The problem statement, all variables and given/known data
    Let g(x) = x^2 + 12x + 36, a parabola with one zero at x = -6. The parabola moves downward at a rate of 2 units/sec. How fast is the distance between the zeroes changing when they are 10 units apart?

    2. Relevant equations
    g(x) = x^2 + 12x + 36

    3. The attempt at a solution
    I figured since this is a parabola, the zeroes would simply be equidistant from -6, so at 10 units apart they would be at x=-1 and x=-11.

    Next I took the derivative of y = x^2 + 12x + 36 to obtain
    dy/dt = (2x + 12)(dx/dt) and I substituted -2 for dy/dt since this parabola is moving down and solved for dx/dt to get
    dx/dt = -2/(2x+12). Next I substituted in -11 and -1 for x and obtained, respectively, dx/dt = 1/5 and -1/5. But this doesn't seem to make sense: doesn't this mean they're moving closer to each other, when they should be going further away? The correct answer is 2/5, so if the signs were what I expected I would've had it correct, but I can't figure out what's wrong.

    It's worth mentioning that I was given a solution that was quite distinct from the manner in which I tried to solve it, but I didn't understand it either. This is the provided solution:
    y = ax^2 + bx +c, therefore dc/dt = -2. Distance between roots = (-b+(b^2-4ac)^(1/2))/(2a) - (-b-(b^2-4ac)^(1/2))/(2a) = ((b^2-4ac)^(1/2))/a = (144-4c)^(1/2). Distance = 10 when c = 11, thus the rate at which they are moving apart is 2/5.

    I understand it up until the last step, where I don't see how the jump was made. Thanks in advance for the help!
  2. jcsd
  3. Feb 15, 2008 #2


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    Science Advisor

    What do you think "the parabola is moving downward at 2 units per second" means? You can't just look at y= x2+ 12x+ 36- that's a single parabola, it isn't moving! A parabola that, at time t= 0, is y= x2+ 12x+ 36 and such that the entire parabola (i.e. y value for every x) is moving downward at 2 units per second must be y= x2+ 12x+ 36- 2t where t is in seconds. That's their "y= ax2+ bx+ c with dc/dt= -2". Here c= 36-2t and dc/dt= -2.

    We can simplify that a bit by noting that x2+ 12x+ 36= (x+ 6)2 so we are really talking about y= (x+6)2- 2t. The zeros are given by (x+6)2= 2t so [itex]x= -6\pm\sqrt{2t}[/itex]. The distance between those zeros is [itex]D= 2\sqrt{2t}[/itex]. You could differentiate that directly but I think it is easier to write D2= 8t and then differentiate: 2D dD/dt= 8 so dD/dt= 4/D. When the zeros are 10 units apart, D= 10 so dD/dt= 4/10= 2/5 unit per second.
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