Problem $2(10$ points$)$

A beam consists of a straight beam AB $($length r $)$ and a $1//4$ circular beam BC $($radius r $).$

The beam is fixed at C and a roller support at B $($either upward or downward force, no

separation$).$ A bending moment M is applied at point A$.$ The bending rigidity of the

beam EI is given.

$($a$)$ Find the reaction force at point B$.$

$($b$)$ Find the rotational angle at point A using energy method.

please help $!$