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<title> 符号表示式的运算 </title>
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<font COLOR="#0000FF">
<h1>11.1.3 符号表示式的运算</h1>
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<p>以下列出几个常用的符号运算函数，它们可以将一符号数学室转换成另一种型态。兹将分列如下：
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<p><font COLOR="#FF0000" FACE="Times New Roman">horner(S)</font> 将<font COLOR="#FF0000" FACE="Times New Roman">S</font>转换成巢状表示式 </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">numden(S)</font> 将<font COLOR="#FF0000" FACE="Times New Roman">S</font>的有理数表示成分子和分母的形式 </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">numeric(S)</font> 将<font COLOR="#FF0000" FACE="Times New Roman">S</font>改成数值式<font FACE="Times New Roman">(</font><font COLOR="#FF0000" FACE="Times New Roman">S</font>不能含有任何符号变数<font FACE="Times New Roman">)</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">poly2sym(c)</font>
转换多项式系数向量<font COLOR="#FF0000" FACE="Times New Roman">c</font>为符号多项式
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<p><font COLOR="#FF0000" FACE="Times New Roman">pretty(S)</font> 将<font COLOR="#FF0000" FACE="Times New Roman">S</font>显示成数学式 </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">sym2poly(S)</font> 转换<font COLOR="#FF0000" FACE="Times New Roman">S</font>为多项式系数向量 </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">symadd(A,B)</font> 执行<font COLOR="#FF0000" FACE="Times New Roman">A+B</font>的符号加法 </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">symdiv(A,B)</font> 执行<font COLOR="#FF0000" FACE="Times New Roman">A+B</font>的符号除法 </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">symmul(A,B)</font> 执行<font COLOR="#FF0000" FACE="Times New Roman">A+B</font>的符号乘法 </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">sympow(S,p)</font> 执行<font COLOR="#FF0000" FACE="Times New Roman">S^p</font>的符号次方运算 </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">symsub(A,B)</font> 执行<font COLOR="#FF0000" FACE="Times New Roman">A+B</font>的符号减法 <br>
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<p>我们接著看几个应用上述函数的例子 </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>>p1 = </font><font COLOR="#FF0000">'</font><font COLOR="#FF0000" FACE="Times New Roman">1/(y-3)</font><font COLOR="#FF0000">'</font><font COLOR="#FF0000" FACE="Times New Roman">;</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>>p2 = </font><font COLOR="#FF0000">'</font><font COLOR="#FF0000" FACE="Times New Roman">3*y/(y+2)</font><font COLOR="#FF0000">'</font><font COLOR="#FF0000" FACE="Times New Roman">;</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>>p3 = </font><font COLOR="#FF0000">'</font><font COLOR="#FF0000" FACE="Times New Roman">(y+4)*(y-3)*y</font><font COLOR="#FF0000">'</font><font COLOR="#FF0000" FACE="Times New Roman">; <br>
</font></p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> symmul(p1,p3)</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">ans=</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">(y+4)*y</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> sympow(p2,3)</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">ans=</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">27*y^3/(y+2)^3</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> symadd(p1,p2)</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">ans=</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">1/(y-3)+3*y/(y+2)</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>>[num,den] = numden(symadd(p1,p2))</font>
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<p><font COLOR="#FF0000" FACE="Times New Roman">ans=</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">[-8*y+2+3*y, (y-3)*(y+2)]</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> horner(symadd(p3,</font><font COLOR="#FF0000">'</font><font COLOR="#FF0000" FACE="Times New Roman">1</font><font COLOR="#FF0000">'</font><font COLOR="#FF0000" FACE="Times New Roman">)</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">ans=</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">1+(-12+(1+y)*y)*y<br>
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