$($Optional Extra Credit$)(+8$ pt $)$ If a shear force $\backslash ($ V$=100\backslash$mathrm$\{$kN$\}\backslash )$ acts on the cross section, determine the shear stress at $($a$)$ point $\backslash ($ A $\backslash )$ and $($b$)$ point $\backslash ($ B $\backslash )$ on the cross section.