به ازای کدام مقدار $a$، تابع با ضابطه $f\left( x \right)=\left\{ \begin{matrix}
\frac{2{{x}^{2}}-x-1}{1-\sqrt{x}}\,\,\,\,\,\,\,\,\,\,\,\,;\,\,\,\,\,\,\,\,x\ne 1 \\
a\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,;\,\,\,\,\,\,\,\,x=1 \\
\end{matrix} \right.$ در نقطه $x=1$ پیوسته است؟