6.
\((1)\)一水平放置的平面图形,用斜二测画法画出了它的直观图,此直观图恰好是一个边长为\(2\)的正方形,则原平面图形的面积为_______________
\((2)A,B\)均为锐角,\({\cos }\left( A+B \right)=-\dfrac{24}{25},{\sin }\left( B+\dfrac{\pi }{3} \right)=\dfrac{3}{5}\),则\({\cos }\left( A-\dfrac{\pi }{3} \right)=\)_______.
\((3)\)已知实数\(x{,}y\)满足不等式组\(\begin{cases} x{-}1{\geqslant }0{,} \\ y{\geqslant }0{,} \\ x{+}y{-}a{\leqslant }0{,} \end{cases}\)若\(z{=}\dfrac{y{-}1}{x{+}1}\)的最大值为\(1\),则正数\(a\)的值为____________
\((4)\)设向量\( \overset{¯}{a}, \overset{¯}{b}, \overset{¯}{c} \)满足\(\left| \overset{¯}{a}\right|=\left| \overset{¯}{b}\right|=2 \), \( \overset{¯}{a}· \overset{¯}{b}=-2 \),\(( \overset{¯}{a}- \overset{¯}{c}, \overset{¯}{b}- \overset{¯}{c})=60^{\circ} \),则\(\left| \overset{¯}{c}\right| \)的最大值等于__________