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<b>let </b><font color="Maroon">c<sub>1</sub></font> be    <a href="incsp_1.html#L1">IncProjStr</a> ;<br/><b>let </b><font color="Maroon">c<sub>2</sub></font>, <font color="Maroon">c<sub>3</sub></font>, <font color="Maroon">c<sub>4</sub></font>, <font color="Maroon">c<sub>5</sub></font> be   <a href="incsp_1.html#NM1">POINT</a> of <font color="Maroon">c<sub>1</sub></font>;<br/><b>let </b><font color="Maroon">c<sub>6</sub></font>, <font color="Maroon">c<sub>7</sub></font> be   <a href="incsp_1.html#NM2">LINE</a> of <font color="Maroon">c<sub>1</sub></font>;<br/>













<b>assume </b><a NAME="E1:21"/><i><font color="Green">E19</font></i>: 
( <font color="Maroon">c<sub>1</sub></font> is <a href="projpl_1.html#V1" target="_self">configuration</a> &amp; <font color="Maroon">c<sub>2</sub></font>,<font color="Maroon">c<sub>3</sub></font> <a href="incproj.html#R3">on</a> <font color="Maroon">c<sub>6</sub></font> &amp; <font color="Maroon">c<sub>4</sub></font>,<font color="Maroon">c<sub>5</sub></font> <a href="incproj.html#R3">on</a> <font color="Maroon">c<sub>7</sub></font> &amp; <font color="Maroon">c<sub>2</sub></font>,<font color="Maroon">c<sub>3</sub></font> <a href="projpl_1.html#R1" target="_self">|'</a> <font color="Maroon">c<sub>7</sub></font> &amp; <font color="Maroon">c<sub>4</sub></font>,<font color="Maroon">c<sub>5</sub></font> <a href="projpl_1.html#R1" target="_self">|'</a> <font color="Maroon">c<sub>6</sub></font> &amp; <font color="Maroon">c<sub>2</sub></font> <a href="hidden.html#NR2" target="_self">&lt;&gt;</a> <font color="Maroon">c<sub>3</sub></font> &amp; <font color="Maroon">c<sub>4</sub></font> <a href="hidden.html#NR2" target="_self">&lt;&gt;</a> <font color="Maroon">c<sub>5</sub></font> )
 ;<br/>

<a NAME="E2:21"/><b>then </b><i><font color="Green">E20</font></i>: 
( <font color="Maroon">c<sub>2</sub></font> <a href="projpl_1.html#NR1" target="_self">|'</a> <font color="Maroon">c<sub>7</sub></font> &amp; <font color="Maroon">c<sub>3</sub></font> <a href="projpl_1.html#NR1" target="_self">|'</a> <font color="Maroon">c<sub>7</sub></font> &amp; <font color="Maroon">c<sub>4</sub></font> <a href="projpl_1.html#NR1" target="_self">|'</a> <font color="Maroon">c<sub>6</sub></font> &amp; <font color="Maroon">c<sub>5</sub></font> <a href="projpl_1.html#NR1" target="_self">|'</a> <font color="Maroon">c<sub>6</sub></font> )
 
<b>by </b><i><a class="ref" href="projpl_1.html#D1" target="_self">Def1</a></i>;<br/>
<a NAME="E3:21"/><b>then </b><i><font color="Green">E21</font></i>: 
<font color="Maroon">c<sub>2</sub></font>,<font color="Maroon">c<sub>3</sub></font>,<font color="Maroon">c<sub>4</sub></font> <a href="projpl_1.html#NR6" target="_self">is_a_triangle</a> 
 
<b>by </b><i><a class="txt" href="projpl_1.html#E1:21"><i><font color="Green">E19</font></i></a>, <a class="ref" href="projpl_1.html#T10" target="_self">Th10</a></i>;<br/>
<a NAME="E4:21"/>
<font color="Maroon">c<sub>4</sub></font>,<font color="Maroon">c<sub>5</sub></font>,<font color="Maroon">c<sub>3</sub></font> <a href="projpl_1.html#NR6" target="_self">is_a_triangle</a> 
 
<b>by </b><i><a class="txt" href="projpl_1.html#E1:21"><i><font color="Green">E19</font></i></a>, <a class="txt" href="projpl_1.html#E2:21"><i><font color="Green">E20</font></i></a>, <a class="ref" href="projpl_1.html#T10" target="_self">Th10</a></i>;<br/>
<a NAME="E5:21"/><b>then </b><i><font color="Green">E22</font></i>: 
<font color="Maroon">c<sub>3</sub></font>,<font color="Maroon">c<sub>4</sub></font>,<font color="Maroon">c<sub>5</sub></font> <a href="projpl_1.html#NR6" target="_self">is_a_triangle</a> 
 
<b>by </b><i><a class="ref" href="projpl_1.html#T11" target="_self">Th11</a></i>;<br/>
<a NAME="E6:21"/><i><font color="Green">E23</font></i>: 
<font color="Maroon">c<sub>4</sub></font>,<font color="Maroon">c<sub>5</sub></font>,<font color="Maroon">c<sub>2</sub></font> <a href="projpl_1.html#NR6" target="_self">is_a_triangle</a> 
 
<b>by </b><i><a class="txt" href="projpl_1.html#E1:21"><i><font color="Green">E19</font></i></a>, <a class="txt" href="projpl_1.html#E2:21"><i><font color="Green">E20</font></i></a>, <a class="ref" href="projpl_1.html#T10" target="_self">Th10</a></i>;<br/>
<a NAME="E7:21"/>
<font color="Maroon">c<sub>2</sub></font>,<font color="Maroon">c<sub>3</sub></font>,<font color="Maroon">c<sub>5</sub></font> <a href="projpl_1.html#NR6" target="_self">is_a_triangle</a> 
 
<b>by </b><i><a class="txt" href="projpl_1.html#E1:21"><i><font color="Green">E19</font></i></a>, <a class="txt" href="projpl_1.html#E2:21"><i><font color="Green">E20</font></i></a>, <a class="ref" href="projpl_1.html#T10" target="_self">Th10</a></i>;<br/>
<a NAME="E8:21"/><b>then </b>
<font color="Maroon">c<sub>5</sub></font>,<font color="Maroon">c<sub>2</sub></font>,<font color="Maroon">c<sub>3</sub></font> <a href="projpl_1.html#NR6" target="_self">is_a_triangle</a> 
 
<b>by </b><i><a class="ref" href="projpl_1.html#T11" target="_self">Th11</a></i>;<br/>
<b>hence </b><a NAME="E9:21"/>
<font color="Maroon">c<sub>2</sub></font>,<font color="Maroon">c<sub>3</sub></font>,<font color="Maroon">c<sub>4</sub></font>,<font color="Maroon">c<sub>5</sub></font> <a href="projpl_1.html#R5" target="_self">is_a_quadrangle</a> 
 <b>by </b><i><a class="txt" href="projpl_1.html#E3:21"><i><font color="Green">E21</font></i></a>, <a class="txt" href="projpl_1.html#E5:21"><i><font color="Green">E22</font></i></a>, <a class="txt" href="projpl_1.html#E6:21"><i><font color="Green">E23</font></i></a>, <a class="ref" href="projpl_1.html#D6" target="_self">Def6</a></i>;<br/>


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