Remove last problem

src/7_differential_equations_1.md

@@ -910,14 +910,5 @@ since we have demonstrated that $\vec{\phi}_{k}(t)$ is a solution of the DE.
     $$\dot{x}_1 = 3 x_1 + x_2$$
     $$\dot{x}_2 = - x_1 + x_2$$  
 
-10. [:sweat:] *Bonus question - A question of this kind will not be on the exam*
-
-    Show that $e^{A t} e^{-At} = I$, where $A$ is an 
-    $n \times n$ matrix and $I$ is the $n \times n$ identity matrix,
-    by using the definition of the matrix exponential in terms of the Taylor series!
-
-
-
-