$1\mathrm{.}1-$ Starting from the basic definition of stiffness, determine the effective stiffness of the combined

$1\mathrm{.}3$ spring and write the equation of motion for the spring$-$mass systems shown in Figs. P$1\mathrm{.}1$ to P$1\mathrm{.}3.$

Figure P$1\mathrm{.}1$

Figure P$1\mathrm{.}3$