6.
已知向量\(a=(2\cos α,\sin ^{2}α)\),\(b=(2\sin α,t)\),\(α∈\left( 0\mathrm{{,}}\dfrac{\pi}{2} \right)\),\(t\)为实数.
\((1)\) 若\(a-b=\left( \dfrac{2}{5}\mathrm{{,}}0 \right)\),求\(t\)的值\(;\)
\((2)\) 若\(t=1\),且\(a·b=1\),求\(\tan \left( 2\alpha{+}\dfrac{\pi}{4} \right)\)的值.