x=x1+x2=ACos2πYt+A2Cos2元t Let:A=A=A x=2AcoS2元 2 Amplit Is change with time ude: slowly Is oscillation factor 2 x x x A t A t 1 2 1 1 2 2 2    cos 2   cos   Let : A1  A2  A x A t t 2 cos 2 2 2 cos 2 2 1  2 1         A t 2 2 cos 2  2  1   Amplit ude ： Is change with time slowly t 2 cos 2  2  1   Is oscillation factor