P$5\mathrm{.}90$ We have a balanced positive$-$sequence three$-$phase source for which

van $($t$)=150$cos$(400$t $+15)$ V

a$.$ Find the frequency of this source in Hz$.$

b$.$ Give expressions for vbn$($t$)$ and vcn$($t$).$

c$.$ Repeat part $($b$)$ for a negative$-$sequence source.