\pgfmathresult值为什么不正确？

coordinate ["$P1$"] (P1) at ($(O1)!\length cm!48.59:(O2)$);
%coordinate ["$P1$"] (P1) at ($(O1)!\length cm!\pgfmathresult:(O2)$);%%%这里的pgfmathresult值为什么不正确？

\documentclass{ctexart}
\usepackage{ amssymb,amsmath, amsthm, amsfonts,thmtools,appendix, bm,framed,graphicx, mathrsfs}
\usepackage{tkz-euclide}
\usetikzlibrary{patterns,arrows,positioning,arrows.meta,calc,fadings,shapes,decorations.markings}

\begin{document}
\begin{tikzpicture}[scale=.6]
\draw[help lines] (0,0) grid (12,12);
\coordinate ["$O_1$"] (O1) at (1,3);
\coordinate ["$O_2$"] (O2) at (5,3);
\draw(O1) circle (1 cm);\draw(O2) circle (2 cm);
\pgfmathsetmacro{\length}{sqrt(7)}
\pgfmathasin{3/4}%\pgfmathresult    %48.59
\coordinate ["$P1$"] (P1) at ($(O1)!\length cm!48.59:(O2)$);
%\coordinate ["$P1$"] (P1) at ($(O1)!\length cm!\pgfmathresult:(O2)$);%%%这里的\pgfmathresult值为什么不正确？
\draw(O1) circle (\length cm);
\coordinate ["$P$"] (P) at ($(P1)!1cm!0:(O2)$);
%\coordinate ["$A$"] (A) at ($(O2)!(P)!(P1)$);
\coordinate ["$B$"] (B) at ($(P)!\length cm!90:(P1)$);
\draw[red] (O2) -- (O1)  (P) -- (B) (O1) -- (P1)--(O2);
\end{tikzpicture}

\pgfmathsetmacro{\length}{sqrt(7)}\pgfmathresult

\pgfmathasin{3/4}\pgfmathresult %48.59

\pgfmathatantwo{3}{sqrt(7)}\pgfmathresult
\end{document}

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