$($From Exercises $11\mathrm{.}3311\mathrm{.}36)$ For each following pair of equivalent formulas, use chains of equivalence to show their equivalence: a$.$x P$($x$)$ Q$($a$)$x $($P$($x$)$ Q$($a$))$ b$.$x P$($x$)$ Q$($a$)$x $($P$($x$)$ Q$($a$))$ c$.$ P$($a$)$x Q$($x$)$x $($P$($a$)$ Q$($x$))$ d$.$ P$($a$)$x Q$($x$)$x $($P$($a$)$ Q$($x$))$