\((1)\)若\(\left( x+a \right){{\left( 1+2x \right)}^{5}}\)的展开式中\({{x}^{3}}\)的系数为\(20\),则\(a=\)__________.
\((2)\)平面向量\( \overset{→}{a}与 \overset{→}{b} \)的夹角为,\(\overrightarrow{a}=\left( 3,4 \right),|\overrightarrow{b}|=1\),则\(|\overrightarrow{a}-2\overrightarrow{b}|=\_\)________.
\((3)\)已知等腰\(Rt\Delta ABC\)中,\(AB=AC=2\),\(D,E\)分别为\(AB,AC\)的中点,沿\(DE\)将\(\Delta ABC\)折成直二面角\((\)如图\()\),则四棱锥\(A-DECB\)的外接球的表面积为__________.
\((4)\)在\(\Delta ABC\)中,角\(A\),\(B\),\(C\)所对的边分别为\(a\),\(b\),\(c\),且满足\(2{{\cos }^{2}}\dfrac{A}{2}=\dfrac{\sqrt{3}}{3}\sin A\),\(\sin (B-C)=4\cos B\sin C\),则\(\dfrac{b}{c}=\)____________.