w = 2·pi·f
U(t) = 0.8
1.41·SIN(w·t) = 0.8
1.41·SIN(2·pi·50·t) = 0.8
SIN(2·pi·50·t) = 0.8/1.41
2·pi·50·t = ASIN(0.8/1.41) + k·2·pi
t1 = (ASIN(0.8/1.41) + k·2·pi)/(2·pi·50) = 0.001920413350 + 0.02·k
t2 = (pi - ASIN(0.8/1.41) + k·2·pi)/(2·pi·50) = 0.008079586649 + 0.02·k