$3-81.$ Figure P$3-81$ shows two rods, $AB$ and $BC,$ support$-$

ing a load at B$.$ Each rod is steel and has a diameter

of $10\mathrm{.}00$ mm $.$ Compute the elongation of rods AB

and BC when a load having a mass of $4200$ kg is sus$-$

pended from $B$;