P$\mathrm{.}4\mathrm{.}3$ Determine the reactions at the built$-$in end of the cantilever beam ABC shown in Fig. P$\mathrm{.}4\mathrm{.}3$ using the principle of virtual work.

Ans. $R_A=3W,M_A=2\mathrm{.}5WL.$

Fig. P$\mathrm{.}4\mathrm{.}3$