$13\mathrm{.}5.$ Show the integration of Eq$.13\mathrm{.}3$ to find Eq$.13\mathrm{.}4.$ Use a definite integration from $r_w$ to $r.$

$13\mathrm{.}6.$ Show the integration of Eq$.13\mathrm{.}4$ over the cross$-$sectional area of the pipe to find Eq$.13\mathrm{.}5.$