利用有限元理论，求一二维梁的固有频率和振型。短边固定，给定长度、横截面积，弹性模量及材料密度已知。并对比理论计算结果进行分析。各参数自己设定。-Using finite element theory, seeking a two-dimensional beam natural frequencies and modes. The short side is fixed, given the length, cross-sectional area, modulus of elasticity