[Latex]PGF and Tikz 中快速地画垂线

\begin{tikzpicture}
\draw [help lines] (0,0) grid (3,2);
%定义坐标点
\coordinate (a) at (0,1);
\coordinate (b) at (3,2);
\coordinate (c) at (2.5,0);
\sqrt{%画线段
\draw[red] (a) -- (b);
%画垂线
\draw[blue,->] (c) -- ($(a)!(c)!(b)$);
\end{tikzpicture}}

垂足定位:
<a>!<c>!<angle>:<b>

首先考虑一条由<a>到<b>的线段,
然后绕点<a>以<angle>角度旋转线段.
然后再过c点作垂线,那么垂足坐标就可以表示为<a>!<c>!<angle>:<b>

如果这条线段没有旋转,则可省略<angle>参数

\begin{tikzpicture}
\draw [help lines] (0,0) grid (3,2);
%定义坐标点
\node[left] (a) at (0,1){$A$};
\node[right]  (b) at (3,2){$B$};
\node [below] (c) at (2.5,0){$C$};
%这是垂足坐标
\node [above] (d) at ($(a)!(c)!(b)$){$D$};
%画线段
\draw[red] (a) -- (b);
%画垂线
\draw[blue,->] (c) -- (d);
\end{tikzpicture}