Theorem
Any 2 - form of the form
\[dx_i \wedge dx_i =0\]
for any arguments \[(\mathbf{a} , \mathbf{b})=((a_1 , a_2, ,,,,a_n),(b_1,b_2,...,b_n)) \]
Proof
\[\begin{equation} \begin{aligned} dx_i \wedge dx_i (\mathbf{a}, \mathbf{b}) &= det \left( \begin{array}{cc} a_i & b_i \\ a_i & b_i \end{array} \right) \\ &- det \left( \begin{array}{cc} dx_i(\mathbf{a}) & dx_i(\mathbf{b}) \\ dx_i(\mathbf{a}) & dx_i(\mathbf{b}) \end{array} \right) \\ &= det \left( \begin{array}{cc} a_i & b_i \\ a_i & b_i \end{array} \right) \\ &=a_i b_i -b_i a_i =0 \end{aligned} \end{equation}\]