@Jake：这是整个文档
\documentclass[11pt, a4paper]{book}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{tikz}
\usepackage{pgfplots}
\usepgflibrary{arrows}
\usepackage{pgf}
\usepackage{Tikz-exp}
\parindent=0pt
\begin{document}
\begin{tikzpicture}
\tikzset{x={(.965cm,-.258cm)}, y={(.7cm,.7cm)}, z={(0cm,1cm)},
sezioni/.style={draw=#1!60, thin, fill=#1!30, fill opacity=0.3},
scale=1.75
}
\coordinate (O) at (0,0,0) node [left] {$O$};
\draw[gray,->] (-2,0,0) -- (2,0,0) node [right, black] {$x$};
\draw[gray, ->] (0,-2,0) -- (0,2,0) node [right, black] {$y$};
\draw[gray, ->] (O) -- (0,0,3) node [left, black] {$z$};
%=====% sezioni %=====%
\foreach \x in {-1,-.85,...,1}
\draw[sezioni=teal] (\x,{sqrt(1-(\x)^2)},0)--(\x,0,{sqrt(4*(1-(\x)^2))})--(\x,{-sqrt(1-(\x)^2)},0)--(\x,{sqrt(1-(\x)^2)},0);
%=====================%
\draw[blue] (O) circle (1);
\draw[red, domain=-1:1, samples=500] plot (\x,0,{sqrt(4*(1-(\x)^2))});
\end{tikzpicture}
\end{document}
x=1 中的红色映射应该产生值零，但它是正数。
答案1
这是由于确定在哪里对函数进行采样时数值不准确造成的。以下代码确保始终对函数的上限进行domain采样：
\documentclass{article}
\usepackage{tikz}
\makeatletter
\def\tikz@plot@samples@recalc#1:#2\relax{%
\pgfmathparse{#1}%
\let\tikz@temp@start=\pgfmathresult%
\pgfmathparse{#2}%
\let\tikz@temp@end=\pgfmathresult%
\pgfmathparse{\tikz@temp@start+(\tikz@temp@end-\tikz@temp@start)/(\tikz@plot@samples-1)}%
\edef\tikz@plot@samplesat{\tikz@temp@start,\pgfmathresult,...,\tikz@temp@end,\tikz@temp@end}%
}
\makeatother
\begin{document}
\begin{tikzpicture}
\draw (-2,0,0) -- (2,0,0);
\draw[red, domain=-1:1, samples=50] plot (\x,{sqrt(4*(1-(\x)^2))});
\end{tikzpicture}
\end{document}