"不需要特征向量就不要去算。

In[1]:= Eigenvalues[{{a, b}, {c, d}}]

Out[1]= {1/2 (a + d - Sqrt[a^2 + 4 b c - 2 a d + d^2]), 

 1/2 (a + d + Sqrt[a^2 + 4 b c - 2 a d + d^2])}

需要就是

In[1]:= Eigensystem[{{a, b}, {c, d}}]

Out[1]= {{1/2 (a + d - Sqrt[a^2 + 4 b c - 2 a d + d^2]), 

  1/2 (a + d + Sqrt[a^2 + 4 b c - 2 a d + d^2])}, {{-((-a + d + Sqrt[

     a^2 + 4 b c - 2 a d + d^2])/(2 c)), 

   1}, {-((-a + d - Sqrt[a^2 + 4 b c - 2 a d + d^2])/(2 c)), 1}}}"