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Homework Help: Particle in Positive Direction

  1. Apr 10, 2012 #1
    1. The problem statement, all variables and given/known data
    d) When is the particle moving in the positive direction?
    [itex]f(t) = cos(πt/4), t ≤ 10[/itex]


    2. Relevant equations
    [itex]f '(t) = -(π/4)sin(π(t)/4)[/itex]


    3. The attempt at a solution
    [itex]0 < -(π/4)sin(π(t)/4)[/itex]

    [itex](π(t)/4)>πn[/itex]

    [itex]t>4n 0<=n<=2[/itex]

    [itex]t>4 t>8[/itex]


    The answer says 8>t>4

    im probably skipping a simple step. wut do?
     
  2. jcsd
  3. Apr 11, 2012 #2

    Mark44

    Staff: Mentor

    So after multiplying both sides by -4/π, you get sin(πt/4) < 0.
    Sketch a graph of y = sin(πt/4) and you'll see that 4 < t < 8 is indeed the interval.

    I don't get what you did to arrive at the inequality below.
     
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