5.12 已知在自由空间传播的均匀平面波的磁场强度为-|||-(z,t)=((e)_(x)+(e)_(y))times 0.8cos (6pi times (10)^8t-2pi z)N/m-|||-(1)求该均匀平面波的频率、波长、相位常数和相速;-|||-(2)求与H(z,t)相伴的电场强度E(z,t);-|||-(3)计算瞬时坡印廷矢量。