In seawater $(=5$ S$/$m $=72,$ and $=1)$ a propagating electric field is given by $6$

$(,)=20$ cos$(210+0\mathrm{.}5)$a$$ V$/$m$.$ Assuming $=0,$ find $($a$)$

and $(10\%)$ and $($b$)$ the instantaneous form of $.$