Using the method of semantic tableaux, determine whether each of the following formulas is valid. If a

formula is not valid, give an example of an interpretation demonstrating this. $($Remember that a formula

A is valid if and only if $$A is unsatisfiable.$)$

$($a$)$ A $=($p $$ q$)($p $$ r$).$

$($b$)$ B $=($p $$ q$)($q $$ p$).$