The airplane was flying horizontally at those times when dy/dt = 0 and
dx/dt = 0. From the given trajectory we have
dy/dt = 3 sin t and
dx/dt = 1 − 3 cos t (6)
Setting dy/dt = 0 yields the equation 3 sin t = 0, or, more simply, sin t = 0. This equation
has four solutions in the time interval 0 ≤ t ≤ 10:
t = 0, t= π, t = 2π, t = 3π
I want to know how do they came up with these four solutions, I solved but I got only 1 t=0
