5.8 由刚度系数均为k的两根弹簧连接三个相同的单摆，单摆摆长为<img src="https://static.zxtiku.com/wangke/imgs/topic/ce7b48059ba935a5db9c4f3adc4d27bc.jpg"/>,质量为m。试求系统的固有频率和模态。<img src="https://storage.tutusouti.com/pzst/question_bank/res/b0/1c/b01cfe06c01c0eb2e99efc8d65e5266df3f38b0c.jpg?x-oss-process=image/sharpen,200,https://static.cnmooc.org/repositry/umeditor/i/1503/28/7c775c846fb8445b86353d4dba9266b3.png"/>