emath.sty の 1行(だけ)を書き直す案です。>対処療法
(要は、適当な高さの右とじ括弧を作るだけです。)

\documentclass[a5j]{jsbook} 
\usepackage{amsmath} 
\usepackage[psamsfonts]{amssymb}% see 美文書 p.142 
\usepackage{pifont} 
\usepackage{multicol}
\usepackage[dvipdfm]{graphicx} 
\usepackage{type1cm}
\usepackage{plext} \usepackage{ascmac,itembbox}%サイコロ
\usepackage[varg,txfonts]{emath} 
\usepackage{bm} 
\usepackage{emathP,emathW,emathZ}
\usepackage{emathPs}% [debug]
\makeatletter
\def\Txzyohou#1#2#3#4{%
    \def\bunkai##1{\global\wari@@cnt\z@\def\t@{}\wari@prfalse
        \@for\@c:=##1\do{\ifx\empty\@c\else\wari@prtrue\fi
        \ifnum\wari@@cnt=\z@\xdef\t@{\t@\@c}\else
        \xdef\t@{\t@&\@c}\fi
        \global\advance\wari@@cnt\@ne
        \ifnum\wari@@cnt=\wari@@cmax
        \ifodd\gyou@@c
            \xdef\wari@@hidari{&\wari@@hidari}
            \ifwari@pr\wari@@hidari\t@\\\fi
        \else
            \global\advance\hidari@@p\@ne
            \ifwari@pr\wari@@hidari\t@\\\cline{\the\hidari@@p-\the\migi@@p}\fi
        \fi
        \global\advance\gyou@@c\@ne\def\t@{}\wari@@cnt\z@\fi
        }}%
    \gyou@@c\z@\hizyo@@c\z@
    \@for\@c:=#1\do{\advance\hizyo@@c\@ne}%
    \zyo@@c\z@
    \@for\@c:=#2\do{\advance\zyo@@c\@ne}%
    \hidari@@p\zyo@@c\advance\hidari@@p\@ne
    \migi@@p\hidari@@p\advance\migi@@p\hizyo@@c
    \def\wari@@hidari{}\wari@@cnt\m@ne
    \@whilenum\wari@@cnt<\zyo@@c\do{\edef\wari@@hidari{&\wari@@hidari}%
        \advance\wari@@cnt\@ne}%
    \wari@@cmax=100\relax
\begin{array}[t]{*{\the\zyo@@c}{@{\,}r}@{\,}l*{\the\hizyo@@c}{@{\,}r}}
    \wari@@hidari\bunkai{#3}\t@\\\cline{\the\hidari@@p-\the\migi@@p}
    \bunkai{#2}\t@&\raisebox{1pt}{$\left.\!{\EMvphantom{\ensuremath{a^{3}}}}\right)$}\ &\bunkai{#1}\t@\\
    \global\wari@@cmax\zyo@@c\bunkai{#4}
    \wari@@hidari&\t@
\end{array}}%
\makeatother
\begin{document}
$\Txzyohou{1x^2,+3x,+5}{1x,+1}{,1x,+2}{1x^2,+1x, 2x,+5, 2x,+2, 3}$

$\zyohou{1x^2,+3x,+5}{1x,+1}{,1x,+2}{1x^2,+1x, 2x,+5, 2x,+2, 3}$
\end{document}

▼関連発言
│
└◆1602:\zyohou のわがままな御願い [tad] 08/20 14:18
　└◆1603:Re:\zyohou のわがままな御願い [tDB] 08/20 15:04
　　└◆1604:Re[2]:\zyohou のわがままな御願い [tad] 08/20 15:36
　　　├◆1605:Re[3]:\zyohou のわがままな御願い [田中徹] 08/20 17:26
　　　│└◆1609:Re[4]:\zyohou のわがままな御願い [tad] 08/20 23:24<-last
　　　└◆1606:Re[3]:\zyohou のわがままな御願い [tDB] 08/20 17:34
　　　　└◆1607:追記 [tDB] 08/20 17:40
　　　　　└◆1608:Re[4]:\zyohou のわがままな御願い+Re:追記 [tad] 08/20 18:22