document.write( "Question 1196342: Find the sum of the 1st 20 terms of the arithmetic sequence
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Algebra.Com's Answer #829132 by ikleyn(52794)\"\" \"About 
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document.write( "First term is  \"a%5B1%5D\" = -2.\r\n" );
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document.write( "The common difference is  d = (-5) - (-2) = -5 + 2 = -3.\r\n" );
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document.write( "Use the formula for the sum of the first n term of an arithmetic progression\r\n" );
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document.write( "    \"S%5Bn%5D\" = \"%28%28a%5B1%5D+%2B+a%5Bn%5D%29%2F2%29%2An\".\r\n" );
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document.write( "You need to know the 20th term  \"a%5B20%5D\". It is (use the formula for the n-th term)\r\n" );
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document.write( "    \"a%5B20%5D\" = \"a%5B1%5D+%2B+%2820-1%29%2Ad\" = -2 + 19*(-3) = -59.\r\n" );
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document.write( "Therefore,  \"S%5B20%5D\" = \"%28%28-2%29+%2B+%28-59%29%29%2F2%29%2A20\" = \"%28%28-61%29%2F2%29%2A20\" = \"%28-61%29%2A10\" = -610.\r\n" );
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document.write( "ANSWER.  The sum of the first 20 terms of this AP is  -610.\r\n" );
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\n" ); document.write( "\n" ); document.write( "For introductory lessons on arithmetic progressions see \r
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\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
\n" ); document.write( "\n" ); document.write( "    - Word 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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