Readings show the helpful voltage dependant on the standard value of samples more than an entire number of durations of the fundamental frequency K?t Qu? X? S? K?t Qu? X? S? Mi?n Nam K?t Qu? X? S? Mi?n Trung K?t Qu? X? S? Mi?n B?c KQXS Các T?nh KQXS ?i?n Toán X? s? ?i?n toán