determine the first three natural frequencies and corresponding mode shapes of a simply supported reinforced concrete beam having a cross section $250$mm wide by $600$ mm deep with a span of $10$m$.$ assume the flexural stiffness of the beam EI$=1\mathrm{.}125*10^8$N$-$M$^2$ and weight per unit volume W $=25$KN$/$M$^3$