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if $\overline{B} = AD$ then $A\overline{B}=\overline{B}$:
$\overline{B} = AD$
$A\overline{B} = AAD$
$A\overline{B} = AD$
$A\overline{B} = \overline{B}$
$\square$
if $\overline{B} = AD$ then $B\overline{A}=\overline{A}$:
$\overline{B} = AD$
$A+\overline{B} = A+AD$
$A+\overline{B} = A$ using absorption laws, which can be proved with a truth table
$\overline{A}B = \overline{A}$
$\square$