The initial position has no effect on the velocity, so you can ignore that value (unless there's another part to the question not included, of course).
We have
[tex]v(t)=v(0)+\displaystyle\int_0^t a(u)\,\mathrm du[/tex]
[tex]v(t)=55+\int_0^t\cos(\pi u)\,\mathrm du[/tex]
[tex]\boxed{v(t)=55+\dfrac1\pi\sin(\pi t)}[/tex]
