the voltage across and current through a certain device are given
v(t)= 5cos(4πt), i(t)= 0.1cos(4πt)
a) the instantanous power p(t) at t=0 andt=0.25s
b) the average power Pav, define as the average value p(t) over a full time period of the cosine function (0 to 0.5s)