$($a$)$ Determine the maximum shearing stress caused by a $40-$kip$\backslash (\backslash$cdot $\backslash )$in$.$ torque $\backslash (\backslash$mathbf$\{$T$\}\backslash )$ in the $3-$in$.-$diameter solid aluminum shaft shown. $($b$)$ Solve part $\backslash ($ a $\backslash ),$ assuming that the solid shaft has been replaced by a hollow shaft of the same outer diameter and of $1-$in$.$ inner diameter.