\((1)\)设直线\(ax-y+3=0\)与圆\({\left(x-1\right)}^{2}+{\left(y-2\right)}^{2}=4 \)相交于\(A\)、\(B\)两点,且弦\(AB\)的长为\(2 \sqrt{3} \),则\(a=\)______.
\((2)\)在\(∆ABC \)中,角\(A\),\(B\),\(C\)所对的边分别为\(a\),\(b\),\(c\),若\(\sin A=2\sin B \),且\(a+b= \sqrt{3}c \),则角\(C\)的大小为______.
\((3)\)已知正四棱锥,其底面边长为\(2\),侧棱长为\(\sqrt{3} \),则该四棱锥外接球的表面积是______.
\((4)\) 在数列\(\left\{{a}_{n}\right\} \)中,\({a}_{1}=1,\left({n}^{2}+n\right)\left({a}_{n+1}-{a}_{n}\right)=2 \),则\({a}_{20}= \)_____.