8.
\((1)\)若复数\(z=(a^{2}+2a-3)+(a+3)i\)为纯虚数\((i\)为虚数单位\()\),则实数\(a\)的值是_____.
\((2)\)已知一个回归方程为\(=1.5x+4.5\),\(x∈\{1,5,7,13,19\}\),则\( \bar{y} =\)__________.
\((3)\)若\(t∈R\),\(t\neq -1\),\(t\neq 0\),复数\(z= \dfrac{t}{1+t}+ \dfrac{1+t}{t} i\)的模的取值范围是__________.
\((4)\)已知\( \sqrt{2+ \dfrac{2}{3}}=2· \sqrt{ \dfrac{2}{3}}, \sqrt{3+ \dfrac{3}{8}}=3· \sqrt{ \dfrac{3}{8}}, \sqrt{4+ \dfrac{4}{15}}=4· \sqrt{ \dfrac{4}{15}}, ….\)
若\( \sqrt{8+ \dfrac{a}{t}}=8· \sqrt{ \dfrac{a}{t}} (a,t\)均为正实数\()\),类比以上等式,可推测\(a\),\(t\)的值,则\(a+t=\)__________.