设各杆质量不计，EI为常数，则图示体系的自振频率为<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/a69a2249a5554633b0ed23df457a4327.png?x-oss-process=image/sharpen,200"/>。（）<br/> <img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/c3e7dc8cfe5c425b87c7c318028e67c9.png?x-oss-process=image/sharpen,200"/>