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<b>let </b><font color="Maroon">c<sub>1</sub></font> be   <a href="nat_1.html#NM1">Nat</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> be   <a href="pre_topc.html#NM2">Point</a> of <span class="p1">(<span class="default"><a href="euclid.html#K15" target="_self">TOP-REAL</a> <font color="Maroon">c<sub>1</sub></font></span>)</span>;<br/>







<b>thus </b><font color="Maroon">c<sub>2</sub></font> <a href="euclid.html#K20" target="_self">-</a> <span class="p1">(<span class="default"><font color="Maroon">c<sub>3</sub></font> <a href="euclid.html#K20" target="_self">-</a> <font color="Maroon">c<sub>4</sub></font></span>)</span> = 
<font color="Maroon">c<sub>2</sub></font> <a href="euclid.html#K20" target="_self">-</a> <span class="p1">(<span class="default"><font color="Maroon">c<sub>3</sub></font> <a href="euclid.html#K17" target="_self">+</a> <span class="p2">(<span class="default"><a href="euclid.html#K19" target="_self">-</a> <font color="Maroon">c<sub>4</sub></font></span>)</span></span>)</span>
<b>by </b><i><a class="ref" href="euclid.html#T45" target="_self">Th45</a></i>
<br/>.= 
<span class="p1">(<span class="default"><font color="Maroon">c<sub>2</sub></font> <a href="euclid.html#K20" target="_self">-</a> <font color="Maroon">c<sub>3</sub></font></span>)</span> <a href="euclid.html#K20" target="_self">-</a> <span class="p1">(<span class="default"><a href="euclid.html#K19" target="_self">-</a> <font color="Maroon">c<sub>4</sub></font></span>)</span>
<b>by </b><i><a class="ref" href="euclid.html#T50" target="_self">Th50</a></i>
<br/>.= 
<span class="p1">(<span class="default"><font color="Maroon">c<sub>2</sub></font> <a href="euclid.html#K20" target="_self">-</a> <font color="Maroon">c<sub>3</sub></font></span>)</span> <a href="euclid.html#K17" target="_self">+</a> <span class="p1">(<span class="default"><a href="euclid.html#K19" target="_self">-</a> <span class="p2">(<span class="default"><a href="euclid.html#K19" target="_self">-</a> <font color="Maroon">c<sub>4</sub></font></span>)</span></span>)</span>
<b>by </b><i><a class="ref" href="euclid.html#T45" target="_self">Th45</a></i>
<br/>.= 
<span class="p1">(<span class="default"><font color="Maroon">c<sub>2</sub></font> <a href="euclid.html#K20" target="_self">-</a> <font color="Maroon">c<sub>3</sub></font></span>)</span> <a href="euclid.html#K17" target="_self">+</a> <font color="Maroon">c<sub>4</sub></font>
<b>by </b><i><a class="ref" href="euclid.html#T39" target="_self">Th39</a></i>
;<br/>


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