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<b>let </b><font color="Maroon">n</font> be    <a href="subset_1.html#M1">Element</a> of  <a href="numbers.html#K5">NAT</a> ;<br/><b>let </b><font color="Maroon">X</font> be   <a href="struct_0.html#NM2">Subset</a> of <span class="p1">(<span class="default"><a href="euclid.html#K15">TOP-REAL</a> <font color="Maroon">n</font></span>)</span>;<br/><b>let </b><font color="Maroon">x</font> be   <a href="pre_topc.html#NM2">Point</a> of <span class="p1">(<span class="default"><a href="euclid.html#K15">TOP-REAL</a> <font color="Maroon">n</font></span>)</span>;<br/><b>let </b><font color="Maroon">y</font> be   <a href="pre_topc.html#NM2">Point</a> of <span class="p1">(<span class="default"><a href="euclid.html#K15">TOP-REAL</a> <font color="Maroon">n</font></span>)</span>;<br/>







<b>thus </b><a NAME="E1:21"/>
<span class="p1">(<span class="default"><font color="Maroon">X</font> <a href="mathmorp.html#K1" target="_self">+</a> <font color="Maroon">x</font></span>)</span> <a href="mathmorp.html#K1" target="_self">+</a> <font color="Maroon">y</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">X</font> <a href="mathmorp.html#K1" target="_self">+</a> <span class="p1">(<span class="default"><font color="Maroon">x</font> <a href="euclid.html#K17">+</a> <font color="Maroon">y</font></span>)</span>
  <i><font color="Red">:: according to </font></i><a class="ref" href="xboole_0.html#D10">XBOOLE_0:def 10</a><div><a class="txt" onclick="hs2(this)" href="javascript:()" title="21_1"><b>proof </b></a><div class="add">

<b>let </b><font color="Maroon">p</font> be    <a href="hidden.html#M1">set</a> ; <i><font color="Red">:: according to </font></i><a class="ref" href="tarski.html#D3">TARSKI:def 3</a><br/>

<b>assume </b><a NAME="E1:21_1"/>
<font color="Maroon">p</font> <a href="hidden.html#R2">in</a> <span class="p1">(<span class="default"><font color="Maroon">X</font> <a href="mathmorp.html#K1" target="_self">+</a> <font color="Maroon">x</font></span>)</span> <a href="mathmorp.html#K1" target="_self">+</a> <font color="Maroon">y</font>
 ;<br/>

<a NAME="E2:21_1"/><b>then </b>
<font color="Maroon">p</font> <a href="hidden.html#R2">in</a> <span class="p1">{<span class="default"> <span class="p2">(<span class="default"><font color="Olive">x1</font> <a href="euclid.html#K17">+</a> <font color="Maroon">y</font></span>)</span> where <font color="Olive">x1</font> is   <a href="pre_topc.html#NM2">Point</a> of <span class="p1">(<span class="default"><a href="euclid.html#K15">TOP-REAL</a> <font color="Maroon">n</font></span>)</span> : <font color="Olive">x1</font> <a href="hidden.html#R2">in</a> <font color="Maroon">X</font> <a href="mathmorp.html#K1" target="_self">+</a> <font color="Maroon">x</font> </span>}</span> 
 
;<br/>
<b>then consider </b><font color="Maroon">x2</font> being   <a href="pre_topc.html#NM2">Point</a> of <span class="p1">(<span class="default"><a href="euclid.html#K15">TOP-REAL</a> <font color="Maroon">n</font></span>)</span><b> such that </b><br/><a NAME="E4:21_1"/><i><font color="Green">E24</font></i>: 
( <font color="Maroon">p</font> <a href="hidden.html#R1">=</a> <font color="Maroon">x2</font> <a href="euclid.html#K17">+</a> <font color="Maroon">y</font> &amp; <font color="Maroon">x2</font> <a href="hidden.html#R2">in</a> <font color="Maroon">X</font> <a href="mathmorp.html#K1" target="_self">+</a> <font color="Maroon">x</font> )
 ;<br/>
<a NAME="E5:21_1"/>
<font color="Maroon">x2</font> <a href="hidden.html#R2">in</a> <span class="p1">{<span class="default"> <span class="p2">(<span class="default"><font color="Olive">x1</font> <a href="euclid.html#K17">+</a> <font color="Maroon">x</font></span>)</span> where <font color="Olive">x1</font> is   <a href="pre_topc.html#NM2">Point</a> of <span class="p1">(<span class="default"><a href="euclid.html#K15">TOP-REAL</a> <font color="Maroon">n</font></span>)</span> : <font color="Olive">x1</font> <a href="hidden.html#R2">in</a> <font color="Maroon">X</font> </span>}</span> 
 
<b>by </b><i/>;<br/>
<b>then consider </b><font color="Maroon">x3</font> being   <a href="pre_topc.html#NM2">Point</a> of <span class="p1">(<span class="default"><a href="euclid.html#K15">TOP-REAL</a> <font color="Maroon">n</font></span>)</span><b> such that </b><br/><a NAME="E7:21_1"/><i><font color="Green">E35</font></i>: 
( <font color="Maroon">x2</font> <a href="hidden.html#R1">=</a> <font color="Maroon">x3</font> <a href="euclid.html#K17">+</a> <font color="Maroon">x</font> &amp; <font color="Maroon">x3</font> <a href="hidden.html#R2">in</a> <font color="Maroon">X</font> )
 ;<br/>
<a NAME="E8:21_1"/>
<font color="Maroon">p</font> <a href="hidden.html#R1">=</a> <font color="Maroon">x3</font> <a href="euclid.html#K17">+</a> <span class="p1">(<span class="default"><font color="Maroon">x</font> <a href="euclid.html#K17">+</a> <font color="Maroon">y</font></span>)</span>
 
<b>by </b><i>, , <a class="ref" href="euclid.html#T30">EUCLID:30</a></i>;<br/>
<a NAME="E9:21_1"/><b>then </b>
<font color="Maroon">p</font> <a href="hidden.html#R2">in</a> <span class="p1">{<span class="default"> <span class="p2">(<span class="default"><font color="Olive">x1</font> <a href="euclid.html#K17">+</a> <span class="p3">(<span class="default"><font color="Maroon">x</font> <a href="euclid.html#K17">+</a> <font color="Maroon">y</font></span>)</span></span>)</span> where <font color="Olive">x1</font> is   <a href="pre_topc.html#NM2">Point</a> of <span class="p1">(<span class="default"><a href="euclid.html#K15">TOP-REAL</a> <font color="Maroon">n</font></span>)</span> : <font color="Olive">x1</font> <a href="hidden.html#R2">in</a> <font color="Maroon">X</font> </span>}</span> 
 
<b>by </b><i/>;<br/>
<b>hence </b><a NAME="E10:21_1"/>
<font color="Maroon">p</font> <a href="hidden.html#R2">in</a> <font color="Maroon">X</font> <a href="mathmorp.html#K1" target="_self">+</a> <span class="p1">(<span class="default"><font color="Maroon">x</font> <a href="euclid.html#K17">+</a> <font color="Maroon">y</font></span>)</span>
 ;<br/>


</div><b>end;</b></div>

<b>let </b><font color="Maroon">p</font> be    <a href="hidden.html#M1">set</a> ; <i><font color="Red">:: according to </font></i><a class="ref" href="tarski.html#D3">TARSKI:def 3</a><br/>

<b>assume </b><a NAME="E2:21"/>
<font color="Maroon">p</font> <a href="hidden.html#R2">in</a> <font color="Maroon">X</font> <a href="mathmorp.html#K1" target="_self">+</a> <span class="p1">(<span class="default"><font color="Maroon">x</font> <a href="euclid.html#K17">+</a> <font color="Maroon">y</font></span>)</span>
 ;<br/>

<a NAME="E3:21"/><b>then </b>
<font color="Maroon">p</font> <a href="hidden.html#R2">in</a> <span class="p1">{<span class="default"> <span class="p2">(<span class="default"><font color="Olive">x1</font> <a href="euclid.html#K17">+</a> <span class="p3">(<span class="default"><font color="Maroon">x</font> <a href="euclid.html#K17">+</a> <font color="Maroon">y</font></span>)</span></span>)</span> where <font color="Olive">x1</font> is   <a href="pre_topc.html#NM2">Point</a> of <span class="p1">(<span class="default"><a href="euclid.html#K15">TOP-REAL</a> <font color="Maroon">n</font></span>)</span> : <font color="Olive">x1</font> <a href="hidden.html#R2">in</a> <font color="Maroon">X</font> </span>}</span> 
 
;<br/>
<b>then consider </b><font color="Maroon">x2</font> being   <a href="pre_topc.html#NM2">Point</a> of <span class="p1">(<span class="default"><a href="euclid.html#K15">TOP-REAL</a> <font color="Maroon">n</font></span>)</span><b> such that </b><br/><a NAME="E5:21"/><i><font color="Green">E36</font></i>: 
( <font color="Maroon">p</font> <a href="hidden.html#R1">=</a> <font color="Maroon">x2</font> <a href="euclid.html#K17">+</a> <span class="p1">(<span class="default"><font color="Maroon">x</font> <a href="euclid.html#K17">+</a> <font color="Maroon">y</font></span>)</span> &amp; <font color="Maroon">x2</font> <a href="hidden.html#R2">in</a> <font color="Maroon">X</font> )
 ;<br/>
<a NAME="E6:21"/><i><font color="Green">E38</font></i>: 
<font color="Maroon">p</font> <a href="hidden.html#R1">=</a> <span class="p1">(<span class="default"><font color="Maroon">x2</font> <a href="euclid.html#K17">+</a> <font color="Maroon">x</font></span>)</span> <a href="euclid.html#K17">+</a> <font color="Maroon">y</font>
 
<b>by </b><i><a class="ref" href="euclid.html#T30">EUCLID:30</a>, </i>;<br/>
<a NAME="E7:21"/>
<font color="Maroon">x2</font> <a href="euclid.html#K17">+</a> <font color="Maroon">x</font> <a href="hidden.html#R2">in</a> <span class="p1">{<span class="default"> <span class="p2">(<span class="default"><font color="Olive">x1</font> <a href="euclid.html#K17">+</a> <font color="Maroon">x</font></span>)</span> where <font color="Olive">x1</font> is   <a href="pre_topc.html#NM2">Point</a> of <span class="p1">(<span class="default"><a href="euclid.html#K15">TOP-REAL</a> <font color="Maroon">n</font></span>)</span> : <font color="Olive">x1</font> <a href="hidden.html#R2">in</a> <font color="Maroon">X</font> </span>}</span> 
 
<b>by </b><i/>;<br/>
<a NAME="E8:21"/><b>then </b>
<font color="Maroon">x2</font> <a href="euclid.html#K17">+</a> <font color="Maroon">x</font> <a href="hidden.html#R2">in</a> <font color="Maroon">X</font> <a href="mathmorp.html#K1" target="_self">+</a> <font color="Maroon">x</font>
 
;<br/>
<a NAME="E9:21"/><b>then </b>
<font color="Maroon">p</font> <a href="hidden.html#R2">in</a> <span class="p1">{<span class="default"> <span class="p2">(<span class="default"><font color="Olive">x1</font> <a href="euclid.html#K17">+</a> <font color="Maroon">y</font></span>)</span> where <font color="Olive">x1</font> is   <a href="pre_topc.html#NM2">Point</a> of <span class="p1">(<span class="default"><a href="euclid.html#K15">TOP-REAL</a> <font color="Maroon">n</font></span>)</span> : <font color="Olive">x1</font> <a href="hidden.html#R2">in</a> <font color="Maroon">X</font> <a href="mathmorp.html#K1" target="_self">+</a> <font color="Maroon">x</font> </span>}</span> 
 
<b>by </b><i/>;<br/>
<b>hence </b><a NAME="E10:21"/>
<font color="Maroon">p</font> <a href="hidden.html#R2">in</a> <span class="p1">(<span class="default"><font color="Maroon">X</font> <a href="mathmorp.html#K1" target="_self">+</a> <font color="Maroon">x</font></span>)</span> <a href="mathmorp.html#K1" target="_self">+</a> <font color="Maroon">y</font>
 ;<br/>


</div>
