This is mycode. How to get `f(sqrt(2))=2*sqrt(2)+9` in this code?

```
\documentclass[12pt]{article}
\usepackage{tikz} 
\usetikzlibrary{fpu}
\usepackage{amsmath}
\begin{document}
\begin{tikzpicture}[%
    >=stealth, scale=1,c/.style={circle,fill,inner sep=1.2pt},
    declare function={a=1;b=3;c=0;d=3;
      f(\x)=a*\x*\x*\x+b*\x*\x+c*\x+d;
    }
  ]
  \path (current bounding box.south) node[below=4pt, fill=yellow!50]{
\begin{minipage}
  {14cm}
\pgfkeys{%
    /pgf/number format/frac,/pgf/number format/frac whole=false,
  }
   \pgfmathparse{1/2}%
   $ f\left(\dfrac{1}{2}\right)=
   \pgfmathprintnumber\pgfmathresult$
   \pgfkeys{%
    /pgf/number format/frac,/pgf/number format/frac whole=true,
  }
   \pgfmathparse{-5}%
   $ f(-5)=
   \pgfmathprintnumber\pgfmathresult$
   \pgfkeys{%
    /pgf/number format/frac,/pgf/number format/frac whole=true,
  }
   \pgfmathparse{-25}%
   $ f(-25)=
   \pgfmathprintnumber\pgfmathresult$
   \pgfkeys{%
    /pgf/number format/frac,/pgf/number format/frac whole=true,
  }
   \pgfmathparse{7/9}%
   $ f(7/9)=
   \pgfmathprintnumber\pgfmathresult$
   
   \pgfkeys{%
    /pgf/number format/frac,/pgf/number format/frac whole=false,
  }
   \pgfmathparse{sqrt(2)}%
   $ f(\sqrt{2})=
   \pgfmathprintnumber\pgfmathresult$
\end{minipage}};
  \end{tikzpicture}
\end{document} 

```