General solutions of the system $X'=\begin{pmatrix} a & b \\ c & d \end{pmatrix} $

Show that all solutions of the system $$X'=\begin{pmatrix} a & b \\ c & d \end{pmatrix} X$$
approach to zero as $t \rightarrow \infty$  if and only if $a+d<0$ and $ad-bc>0$.

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