1.
设向量\( \overrightarrow{OA}=(a,\cos 2x)\),\( \overrightarrow{OB}=(1+\sin 2x,1)\),\(x∈R\),函数\(f(x)= \begin{vmatrix} \overset{ \overrightarrow{OA}}{\;}\end{vmatrix} ⋅ \begin{vmatrix} \overset{ \overrightarrow{OB}}{\;}\end{vmatrix} \cos ∠AOB\)
\((\)Ⅰ\()\)当\(y=f(x)\)的图象经过点\(( \dfrac {π}{4},2)\)时,求实数\(a\)的值;
\((\)Ⅱ\()\)在\((\)Ⅰ\()\)的条件下,若\(x\)为锐角,当\(\sin ^{2}x=\sin ( \dfrac {π}{4}+α)⋅\sin ( \dfrac {π}{4}-α)+ \dfrac {1-\cos 2α}{2}\)时,求\(\triangle OAB\)的面积;
\((\)Ⅲ\()\)在\((\)Ⅰ\()\)的条件下,记函数\(h(x)=f(x+t)(\)其中实数\(t\)为常数,且\(0 < t < π).\)若\(h(x)\)是偶函数,求\(t\)的值.