We derive explicit formulas for calculating $e^A$, $cosh{A}$, $sinh{A}, cos{A}$ and $sin{A}$ for a given $2times2$ matrix $A$. We also derive explicit formulas for $e^A$ for a given $3times3$ matrix $A$. These formulas are expressed exclusively in te
rms of the characteristic roots of $A$ and involve neither the eigenvectors of $A$, nor the transition matrix associated with a particular canonical basis. We believe that our method has advantages (especially if applied by non-mathematicians or students) over the more conventional methods based on the choice of canonical bases. We support this point with several examples for solving first order linear systems of ordinary differential equations with constant coefficients.
