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A machine part is undergoing SHM with a frequency of 5.00Hz and amplitude 1.80cm . How long does it take the part to gofrom x=0 to x= -1.80 cm

well i know that x=Acos(ωt)
and

ω=2πf so we have -1.8cm= 1.8cm * cos(2πft)

we end up with -1=cos(2πft) I know that cos(π)=-1so ft =1/2 but if that is true

then I have 5s^-1*t= 1/2 gives t= .1s

not .05s like the book says can anyone help me with the problem I'mhaving.