document.write( "Question 1093154: 1+2+3...+81= solve it
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Algebra.Com's Answer #707755 by ikleyn(52781)\"\" \"About 
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document.write( "1 + 2 + 3 + . . . + n = the sum of the first n natural numbers = \r\n" );
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document.write( "                      = the sum of the first n terms of an Arithmetic progression\r\n" );
document.write( "                        with the first term 1 and the common difference 1 =\r\n" );
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document.write( "                      = \"%28n%2A%28n%2B1%29%29%2F2\".\r\n" );
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document.write( "In your case n = 81 and the sum is  \"%2881%2A82%29%2F2\" = 81*41 = 3321.\r\n" );
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document.write( "Answer. 3321.\r\n" );
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\n" ); document.write( "For introductory lessons on Arithmetic progressions see\r
\n" ); document.write( "\n" ); document.write( "    - Arithmetic progressions\r
\n" ); document.write( "\n" ); document.write( "    - The proofs of the formulas for arithmetic progressions \r
\n" ); document.write( "\n" ); document.write( "    - Problems on arithmetic progressions \r
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\n" ); document.write( "\n" ); document.write( "Also,  you have this free of charge online textbook in ALGEBRA-II in this site\r
\n" ); document.write( "\n" ); document.write( "    - ALGEBRA-II - YOUR ONLINE TEXTBOOK.\r
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\n" ); document.write( "\n" ); document.write( "The referred lessons are the part of this online textbook under the topic \"Arithmetic progressions\".\r
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\n" ); document.write( "\n" ); document.write( "Save the link to this textbook together with its description\r
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\n" ); document.write( "\n" ); document.write( "Free of charge online textbook in ALGEBRA-II
\n" ); document.write( "https://www.algebra.com/algebra/homework/complex/ALGEBRA-II-YOUR-ONLINE-TEXTBOOK.lesson\r
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\n" ); document.write( "\n" ); document.write( "into your archive and use when it is needed.\r
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