9.已知有一简谐振动 _(1)=Acos (5t+dfrac (pi )(5)), 另有一个同方向简谐振动 _(2)=Acos (5t+varphi ), 若-|||-两个振动合成振幅最小,则φ的取值应为 ()-|||-A dfrac (pi )(5) B dfrac (4pi )(5) C -dfrac (4pi )(5) D -dfrac (pi )(5)-|||-为据劝总能量的 ()