一质点同时参与两个同方向、同频率的简谐振动，两个振动的表达式分别为_(1)=5cos (4pi t+dfrac (pi )(3))，_(1)=5cos (4pi t+dfrac (pi )(3))，式中_(1)=5cos (4pi t+dfrac (pi )(3))的单位为_(1)=5cos (4pi t+dfrac (pi )(3))，_(1)=5cos (4pi t+dfrac (pi )(3))的单位为_(1)=5cos (4pi t+dfrac (pi )(3))，其合振动的表达式为_(1)=5cos (4pi t+dfrac (pi )(3))________。