已知集合\(A=\left\{ x|(x-6)(x-2a-5) > 0 \right\}\),\(B=\left\{ x|\left[ ({{a}^{2}}+2)-x \right]\cdot (2a-x) < 0 \right\}\).
\(⑴\)若\(a=5\),求\(f(x)\);
\(⑵\)已知\(a > \dfrac{1}{2}.\)且“\(x\in A\)”是“\(f(x)\)”的必要不充分条件,求实数\(a\)的取值范围.
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