$1\mathrm{.}40$ A $20-$mm$-$diameter rod made of the same material as rods $AC$ and $AD$ in the truss shown was tested to failure and an ultimate load of $130$ kN was recorded. Using a factor of safety of $3\mathrm{.}0,$ determine the required diameter $(a)$ of rodAC,$(b)$ of rodAD.