document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=1093154>Question 1093154</A>: <I><SPAN STYLE=\"word-break: break-all;\"> 1+2+3...+81=  solve it<BR>\n" );
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document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #707765 by </B><A HREF=/tutors/aboutme.mpl?userid=Alan3354><B>Alan3354(69443)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=Alan3354><IMG SRC=http://www.algebra.com/images/aboutme-small.gif BORDER=0 alt=\"About Me\" VALIGN=MIDDLE></A>&nbsp;<IMG SRC=http://www.algebra.com/cgi-bin/show-face.mpl?user=Alan3354><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=707765\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> 1+2+3...+81=  solve it<BR>\n" );
document.write( "1+2+3...+80 --> 80 numbers<BR>\n" );
document.write( "Make 40 pairs:<BR>\n" );
document.write( "1 + 80 = 81<BR>\n" );
document.write( "2 + 79 = 81<BR>\n" );
document.write( "etc<BR>\n" );
document.write( "40*81 = 3240<BR>\n" );
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document.write( "Add 81<BR>\n" );
document.write( "--> 3321<BR>\n" );
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document.write( "If you prefer remembering rules and formulas:<BR>\n" );
document.write( "Sum = n*(n+1)/2 = 81*41<BR>\n" );
document.write( "= 3321<BR>\n" );
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