a particle moves along the x-axis in such a way that at time t>0 its position is x=sin(e^t)

at what time does the particle have a velocity of zero?

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I take the derivative and get (e^t)*cos(e^t)

so when does that equal zero?

e^t is never zero, right?

so when does cos(e^t) equal zero?

I'm not sure how to solve a trig function with a variable argument.

help?