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<TITLE>  符号表示式 </TITLE>
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<H1>11.1.1  符号表示式</H1>
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在<FONT FACE="Times New Roman">MATLAB</FONT>中是将一符号表示式储存唯一字串<FONT FACE="Times New Roman">
(character string)</FONT>，即是以二个单引号之内的表示式来定义其为
一符号式，例如<FONT FACE="Times New Roman">
</FONT><FONT COLOR=#FF0000>'</FONT><FONT COLOR=#FF0000 FACE="Times New Roman">tan(y/x)</FONT><FONT COLOR=#FF0000>'</FONT><FONT FACE="Times New Roman">,</FONT><FONT COLOR=#FF0000 FACE="Times New Roman">
</FONT><FONT COLOR=#FF0000>'</FONT><FONT COLOR=#FF0000 FACE="Times New Roman">x^3
- 2*x^2 + 3</FONT><FONT COLOR=#FF0000>'</FONT><FONT FACE="Times New Roman">,</FONT><FONT COLOR=#FF0000 FACE="Times New Roman">
</FONT><FONT COLOR=#FF0000>'</FONT><FONT COLOR=#FF0000 FACE="Times New Roman">1/(cos(angle)+2)</FONT><FONT COLOR=#FF0000>'</FONT><FONT FACE="Times New Roman">
</FONT>的三个式子。<BR>
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在一符号表示式中，需要定义所谓的独立变数。如果未曾事先指定何者为独立变数，<FONT FACE="Times New Roman">MATLAB</FONT>会自行决定。
而它所决定变数的原则如下：它会挑选一个除了<FONT COLOR=#FF0000 FACE="Times New Roman">i</FONT>和<FONT COLOR=#FF0000 FACE="Times New Roman">j</FONT>之外而在字母上最接近<FONT COLOR=#FF0000 FACE="Times New Roman">x</FONT>的小写字元；如果在式子中并无
上述字元，则<FONT COLOR=#FF0000 FACE="Times New Roman">x</FONT>会被视为预设的独立变数。函数<FONT COLOR=#FF0000 FACE="Times New Roman">symvar(s)</FONT><FONT FACE="Times New Roman">
</FONT>可以用已决定何者为独立变数。请看以下的例子：<BR>
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expression S   symvar(S)
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<FONT COLOR=#FF0000>'</FONT><FONT COLOR=#FF0000 FACE="Times New Roman">tan(y/x)</FONT><FONT COLOR=#FF0000>'
   </FONT><FONT COLOR=#FF0000 FACE="Times New Roman">x</FONT>
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<FONT COLOR=#FF0000>'</FONT><FONT COLOR=#FF0000 FACE="Times New Roman">x^3-2*x^2+3</FONT><FONT COLOR=#FF0000>'
 </FONT><FONT COLOR=#FF0000 FACE="Times New Roman">x</FONT>
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<FONT COLOR=#FF0000>'</FONT><FONT COLOR=#FF0000 FACE="Times New Roman">1/(cos(angle)+2)</FONT><FONT COLOR=#FF0000>'
 </FONT><FONT COLOR=#FF0000 FACE="Times New Roman">x</FONT>
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<FONT COLOR=#FF0000>'</FONT><FONT COLOR=#FF0000 FACE="Times New Roman">3*a*b-6</FONT><FONT COLOR=#FF0000>'
   </FONT><FONT COLOR=#FF0000 FACE="Times New Roman">b<BR>
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MATLAB提供了一个函数<FONT COLOR=#FF0000 FACE="Times New Roman">ezplot</FONT><FONT FACE="Times New Roman">
</FONT>可以画单变数的符号式，其预设的独立变数的范围是<IMG SRC="img00003-8.gif" tppabs="http://166.111.167.223/computer/cai/matlabjc/img11/img00003.gif">。它的语法为<FONT FACE="Times New Roman">
</FONT><FONT COLOR=#FF0000 FACE="Times New Roman">ezplot(S)</FONT><FONT FACE="Times New Roman">,
</FONT><FONT COLOR=#FF0000 FACE="Times New Roman">S</FONT>代表符号变数；另一各相关语法<FONT FACE="Times New Roman">
</FONT><FONT COLOR=#FF0000 FACE="Times New Roman">ezplot(S,[xmin,xmax])</FONT>，则是设定独立变数的范围<FONT COLOR=#FF0000 FACE="Times New Roman">xmin</FONT>到<FONT COLOR=#FF0000 FACE="Times New Roman">xmax</FONT>。<HR>
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