Solving Differentiation Help: Find Time When Acceleration = 0

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Differentiation Help!

Homework Statement


A particle is moving along a straight line so that, at time t seconds after leaving a fixed point O, its velocity vms-1 is given by v=10sin(1/2 t).
Find the time when the acceleration, given by dv/dt, is first zero.

Homework Equations


sinx differentiates to cosx

The Attempt at a Solution


Ok i know they are asking me to find the time where there is a stationary point so therefore the gradient or dv/dt = 0.
From differentiating 10sin(1/2 t) i got 5cos(1/2 t). so therefore, 5cos(1/2 t)=0 so i thought you multiply by 2 to get rid of the half then divide by 10 so the Right hand side= 0 and then inverse cos of 0 = 1/2 Pi but the answer in the textbook gives Pi.
Please help me any body!
 
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ohhh yeah of course! thank you very much =]