Problem $1(15$ points$)$: For each of the following, use only the tree method to either show that the

argument is valid, or else find a counterexample.

a$.$

$A=>B$

notC$=>A$

:$.$not$(B=>C)$

b$.,(A^{{?}^{?}}B)=>(CvvD)$

notC$=>A$

$(not{C}^{{?}^{?}}D)=>(E^{{?}^{?}}$notE$)$

$DvvB$

:$.(EvvnotE{)}^{{?}^{?}}C$