A structure is loaded as illustrated. The triangular plate BCDF is assumed to be rigid. $AB$ is steel $)=(200$GPa and has a cross$-$sectional area, $A_{AB}=310m{m}^2.ED$ is aluminum $)=(70$GPa and has a cross$-$sectional area of $620m{m}^2.$ Determine the stress in AB and ED$.$

$($Solution: ${}_{AB}=7\mathrm{.}63$MPa,${}_{DE}=10$MPa $)$