A copper strip $($Ec$=105$ GPa$)$ and an aluminum strip $($Ea $=75$ GPa$)$ are bonded together to form the composite beam shown. Knowing that the beam is bent about a horizontal axis by a couple of moment M $=35$ N$*$m$,$ determine the maximum stress in $($a$)$ the aluminum strip, $($b$)$ the copper strip. $($answers a $=-56\mathrm{.}0$ MPa, b $=66\mathrm{.}4$ MPa$)$