% ============================================================
% Grafik Fungsi Kuadrat dengan TikZ - pendamping artikel blog
% https://pakgun.gezytech.web.id/tikz-grafik-fungsi-kuadrat/
% Kompilasi: pdflatex (butuh paket tikz)
% ============================================================
\documentclass{article}
\usepackage[a4paper, margin=2cm]{geometry}
\usepackage{tikz}

\begin{document}

\section*{1. Grafik $f(x) = x^2 - 4x + 1$}

Perhatikan: \texttt{domain} menjaga kurva di dalam bidang,
\texttt{samples=100} menjaga kurva tetap mulus.

\begin{tikzpicture}[scale=0.7]
  % sumbu
  \draw[->] (-2,0) -- (6,0) node[right] {$x$};
  \draw[->] (0,-4) -- (0,6) node[above] {$y$};
  \foreach \x in {1,...,5}
    \draw (\x,0.1) -- (\x,-0.1) node[below] {\footnotesize \x};
  \foreach \y in {-3,...,5}
    \draw (0.1,\y) -- (-0.1,\y) node[left] {\footnotesize \y};
  % parabola
  \draw[thick, blue, domain=-1:5, samples=100]
    plot (\x, {\x*\x - 4*\x + 1});
  % titik puncak
  \filldraw[red] (2,-3) circle (2pt)
    node[below right] {$(2, -3)$};
\end{tikzpicture}

\section*{2. Contoh lain: $g(x) = x^2 - 2x - 3$}

Diskriminan $D = 4 + 12 = 16$, titik potong sumbu-$x$: $x=-1$ dan $x=3$.
Domain dibuat lega ($-2$ sampai $4$) supaya kedua titik potong ``bernafas'':

\begin{tikzpicture}[scale=0.7]
  \draw[->] (-3,0) -- (5,0) node[right] {$x$};
  \draw[->] (0,-4.5) -- (0,6) node[above] {$y$};
  \foreach \x in {-2,...,4}
    \draw (\x,0.1) -- (\x,-0.1) node[below] {\footnotesize \x};
  \foreach \y in {-4,...,5}
    \draw (0.1,\y) -- (-0.1,\y) node[left] {\footnotesize \y};
  \draw[thick, blue, domain=-1.6:3.6, samples=100]
    plot (\x, {\x*\x - 2*\x - 3});
  \filldraw[red] (1,-4) circle (2pt) node[below right] {$(1, -4)$};
  \filldraw[red] (-1,0) circle (2pt) node[above left] {$(-1, 0)$};
  \filldraw[red] (3,0) circle (2pt) node[above right] {$(3, 0)$};
\end{tikzpicture}

\end{document}