7.
如图,在平面直角坐标系\(xoy\)中,点\(A\left( {{x}_{1}},{{y}_{1}} \right)\),\(B\left( {{x}_{2}},{{y}_{2}} \right)\)在单位圆上,\(\angle xOA=\alpha \),\(α∈\left( \dfrac{π}{6}, \dfrac{π}{2}\right) \),\(\angle AOB=\dfrac{\pi }{3}\).
\((1)\)若\({\cos }\left( \alpha +\dfrac{\pi }{4} \right)=-\dfrac{3}{5}\),求\({{x}_{1}}\)的值;
\((2)\)过点\(A\)作\(x\)轴的垂线交单位圆于另一点\(C\),过\(B\)作\(x\)轴的垂线,垂足为\(D\),记\(\Delta AOC\)的面积为\({{S}_{1}}\),\(\Delta BOD\)的面积为\({{S}_{2}}\),设\(f\left( \alpha \right)={{S}_{1}}+{{S}_{2}}\),求函数\(f\left( \alpha \right)\)的最大值.