# tkz-fct: error message when plotting tangent line

I tried to plot tangent lines to the graph of a function using tkz-fct. See the example below. However I get the following error message:

ERROR: Use of \tkz@DrawTangentLine doesn't match its definition.

--- TeX said ---
\pgfutil@ifnextchar ...1\def \pgfutil@reserved@a {
#2}\def \pgfutil@reserved@...
l.17       \tkzDrawTangentLine[draw](4)


Any idea what's wrong with my code?

The example:

\documentclass{article}
\usepackage{tikz}
\usepackage{tkz-fct}
\usetkzobj{all}

\begin{document}

\begin{tikzpicture}[xscale=1]
\tikzset{tan style/.style={-}}
\tkzInit[xmin=0,xmax=10,xstep=1,
ymin=0,ymax=5,ystep=1]
\tkzGrid[color=brown,sub,subxstep=50,subystep=200]
\tkzAxeXY
\tkzFct[color=red,samples=100,domain = 0:10]{sqrt(1/2.*\x) + sin(1/2.*\x)}
\tkzDrawTangentLine[draw](4)
\end{tikzpicture}
\end{document}

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First you don't need usetkzobj. This is for tkz-euclide. Now the problem comes from sqrt, a possibility is

\documentclass{article}
\usepackage{tikz}
\usepackage{tkz-fct}

\begin{document}

\begin{tikzpicture}[xscale=1]
\tikzset{tan style/.style={-}}
\tkzInit[xmin=0,xmax=10,xstep=1,
ymin=0,ymax=5,ystep=1]
\tkzGrid[color=brown,sub,subxstep=50,subystep=200]
\tkzAxeXY
\tkzFct[color=red,samples=100,domain = 0:10]{(1./2*\x)**(0.5) + sin(1./2*\x)}
\tkzDrawTangentLine[draw](4)
\end{tikzpicture}
\end{document}


Explanation : The code is very tricky : first the syntax is the gnuplot's syntax to draw the graph but then to draw some points and the tangent line, I use FP. So I created a macro to translate the gnuplot expression with the syntax of FP. When I wrote tkz-fct, I did not know how to use gnuplot to calculate a single value, now I know how to make this ... but with Lua, perhaps it's preferable to wait some time and to write new macros with luatex. In the case of sqrt`, I think I need to modify something because this is a bug !

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