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<b>let </b><font color="Maroon">G</font> be    <a href="incsp_1.html#L1">IncProjStr</a> ;<br/><b>let </b><font color="Maroon">a1</font> be   <a href="incsp_1.html#NM1">POINT</a> of <font color="Maroon">G</font>;<br/><b>let </b><font color="Maroon">a2</font> be   <a href="incsp_1.html#NM1">POINT</a> of <font color="Maroon">G</font>;<br/><b>let </b><font color="Maroon">b1</font> be   <a href="incsp_1.html#NM1">POINT</a> of <font color="Maroon">G</font>;<br/><b>let </b><font color="Maroon">b2</font> be   <a href="incsp_1.html#NM1">POINT</a> of <font color="Maroon">G</font>;<br/><b>let </b><font color="Maroon">A</font> be   <a href="incsp_1.html#NM2">LINE</a> of <font color="Maroon">G</font>;<br/><b>let </b><font color="Maroon">B</font> be   <a href="incsp_1.html#NM2">LINE</a> of <font color="Maroon">G</font>;<br/>













<b>assume </b><a NAME="E1:21"/><i><font color="Green">E36</font></i>: 
( <font color="Maroon">G</font> is <a href="projpl_1.html#V1" target="_self">configuration</a> &amp; <font color="Maroon">a1</font>,<font color="Maroon">a2</font> <a href="incproj.html#R3">on</a> <font color="Maroon">A</font> &amp; <font color="Maroon">b1</font>,<font color="Maroon">b2</font> <a href="incproj.html#R3">on</a> <font color="Maroon">B</font> &amp; <font color="Maroon">a1</font>,<font color="Maroon">a2</font> <a href="projpl_1.html#R1" target="_self">|'</a> <font color="Maroon">B</font> &amp; <font color="Maroon">b1</font>,<font color="Maroon">b2</font> <a href="projpl_1.html#R1" target="_self">|'</a> <font color="Maroon">A</font> &amp; <font color="Maroon">a1</font> <a href="hidden.html#NR2" target="_self">&lt;&gt;</a> <font color="Maroon">a2</font> &amp; <font color="Maroon">b1</font> <a href="hidden.html#NR2" target="_self">&lt;&gt;</a> <font color="Maroon">b2</font> )
 ;<br/>

<a NAME="E2:21"/><b>then </b><i><font color="Green">E37</font></i>: 
( <font color="Maroon">a1</font> <a href="projpl_1.html#NR1" target="_self">|'</a> <font color="Maroon">B</font> &amp; <font color="Maroon">a2</font> <a href="projpl_1.html#NR1" target="_self">|'</a> <font color="Maroon">B</font> &amp; <font color="Maroon">b1</font> <a href="projpl_1.html#NR1" target="_self">|'</a> <font color="Maroon">A</font> &amp; <font color="Maroon">b2</font> <a href="projpl_1.html#NR1" target="_self">|'</a> <font color="Maroon">A</font> )
 
<b>by </b><i/>;<br/>
<a NAME="E3:21"/><b>then </b><i><font color="Green">E40</font></i>: 
<font color="Maroon">a1</font>,<font color="Maroon">a2</font>,<font color="Maroon">b1</font> <a href="projpl_1.html#NR6" target="_self">is_a_triangle</a> 
 
<b>by </b><i>, </i>;<br/>
<a NAME="E4:21"/>
<font color="Maroon">b1</font>,<font color="Maroon">b2</font>,<font color="Maroon">a2</font> <a href="projpl_1.html#NR6" target="_self">is_a_triangle</a> 
 
<b>by </b><i>, , </i>;<br/>
<a NAME="E5:21"/><b>then </b><i><font color="Green">E41</font></i>: 
<font color="Maroon">a2</font>,<font color="Maroon">b1</font>,<font color="Maroon">b2</font> <a href="projpl_1.html#NR6" target="_self">is_a_triangle</a> 
 
<b>by </b><i/>;<br/>
<a NAME="E6:21"/><i><font color="Green">E42</font></i>: 
<font color="Maroon">b1</font>,<font color="Maroon">b2</font>,<font color="Maroon">a1</font> <a href="projpl_1.html#NR6" target="_self">is_a_triangle</a> 
 
<b>by </b><i>, , </i>;<br/>
<a NAME="E7:21"/>
<font color="Maroon">a1</font>,<font color="Maroon">a2</font>,<font color="Maroon">b2</font> <a href="projpl_1.html#NR6" target="_self">is_a_triangle</a> 
 
<b>by </b><i>, , </i>;<br/>
<a NAME="E8:21"/><b>then </b>
<font color="Maroon">b2</font>,<font color="Maroon">a1</font>,<font color="Maroon">a2</font> <a href="projpl_1.html#NR6" target="_self">is_a_triangle</a> 
 
<b>by </b><i/>;<br/>
<b>hence </b><a NAME="E9:21"/>
<font color="Maroon">a1</font>,<font color="Maroon">a2</font>,<font color="Maroon">b1</font>,<font color="Maroon">b2</font> <a href="projpl_1.html#R5" target="_self">is_a_quadrangle</a> 
 <b>by </b><i>, , , </i>;<br/>


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