$3\mathrm{.}6$ Simplify the following Boolean expressions, using four$-$variable K$-$maps:

$($a$)*A\text{'}BCD+ACD+BCD+$A'BCD$+BCD$

$($b$){?}^{**},x\text{'}z+w\text{'}xy\text{'}+w(\frac{x}{y}+xy\text{'})$