在\(\triangle ABC\)中,角\(A\),\(B\),\(C\)的对边分别为\(a\),\(b\),\(c\),已知\(b\cos ^{2} \dfrac {A}{2}+a\cos ^{2} \dfrac {B}{2}= \dfrac {3}{2}c.\)
\((\)Ⅰ\()\)求证:\(a\),\(c\),\(b\)成等差数列;
\((\)Ⅱ\()\)若\(C= \dfrac {π}{3}\),\(\triangle ABC\)的面积为\(2 \sqrt {3}\),求\(c\).