Źródło TeX:

p(t)=u(t)i(t)=U_mI_msin{(}\omega t)sin{(}\omega t-\phi)=\frac{U_mI_m}{2}\left[cos{\phi}-cos{(}2\omega t-\phi)\right]=\\ =\left|U\right|\left|I\right|[cos{-}cos{(}2\omega t-\phi)]