x $=$ A cos $(1\mathrm{.}05$t $+3\mathrm{.}67)$

Find the $3$ first moments $($t $>0)$ for which: $($b$)$ Vx$=-0\mathrm{.}5$ Vmax

$($Vmax is the maximum speed$)$

two of the answers are $5$ and $9$