Theory: Readings demonstrate the productive voltage determined by the common worth of samples about a whole quantity of intervals of the elemental 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 t