TikZ로 그림을 그리는 것을 잊지 않기 위해 필기를 합니다. - 잘못 썼나 봐요.

\documentclass[12pt,dvipdfmx]{jsarticle}
\usepackage{tikz}
\usetikzlibrary{angles,quotes}
\begin{tikzpicture}
\coordinate(A) at(0,0);
\coordinate(B) at(3,0);
\coordinate(C) at(1.5,2.5);
\pic [draw=black, angle eccentricity=1.8, angle radius=1cm, "$\theta$"] {angle=B--A--C};
\end{tikzpicture}

\documentclass[12pt,dvipdfmx]{standalone}
\usepackage{tikz}
\usetikzlibrary{patterns,intersections,calc,angles,quotes}

\begin{document}
\begin{tikzpicture}
    \draw [name path=xaxis,->](-0.5,0)--(3,0);
    \draw [name path=yaxis,->](0,-0.5)--(0,3);
    \node [anchor=south] at(0,3) {$y$};
    \node [anchor=west] at(3,0) {$x$};
    \path [name intersections={of=xaxis and yaxis,by=in}];
    \coordinate (O) at(in);     %x軸とy軸の交点をOとする
    \node [anchor=north east] at(O) {O};
    \draw [name path=la, domain=-0.5:2] plot(\x,\x+1);
    \draw [name path=lb, domain=-0.5:2.3] plot(\x,-\x+2);
    \path [name intersections={of=la and lb,by=in2}];　%laとlbの交点をin2とする
    \node [anchor=west] at(in2) {A};　　%in2にAという文字列を配置
\end{tikzpicture}
\end{document}