\input amstex

$$\aligned
Ax+By&=0\\
Cx+Dy&=0
\endaligned\;\Longrightarrow\;y=\frac{AD-BC}{B}x\;\Longrightarrow\;\biggl((x,y)\neq(0,0)\;\Longleftrightarrow\;\vmatrix
A&B\\C&D
\endvmatrix=0\biggr)$$
\bye