document.write( "Question 1109076: If the 4
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Algebra.Com's Answer #724090 by ikleyn(52780)\"\" \"About 
You can put this solution on YOUR website!
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document.write( "the 4-th term is  \"a%5B4%5D\" = \"a%5B1%5D+%2B+3%2Ad\" = 16    (1)\r\n" );
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document.write( "the 8-th term is  \"a%5B8%5D\" = \"a%5B1%5D+%2B+7%2Ad\" = 28.   (2)\r\n" );
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document.write( "Subtract eq(1) from eq(2). You will get\r\n" );
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document.write( "    7d - 3d = 28 - 16 = 12  ====>  4d = 12  ====>  d = \"12%2F4\" = 3.\r\n" );
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document.write( "Thus you just found the common difference d. It is 3.\r\n" );
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document.write( "Next, from eq(1)  \"a%5B1%5D\" = 16 - 3d = 16 - 3*3 = 7.\r\n" );
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document.write( "Thus, the first term  \"a%5B1%5D\" = 7.\r\n" );
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document.write( "Then the 16-term is  \"a%5B16%5D\" = \"a%5B1%5D%2B15%2Ad\" = 7 + 15*3 = 52.\r\n" );
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\n" ); document.write( "There is a bunch of lessons on arithmetic progressions in this site:\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
\n" ); document.write( "\n" ); document.write( "    - Word problems on arithmetic progressions\r
\n" ); document.write( "\n" ); document.write( "    - Mathematical induction and arithmetic progressions\r
\n" ); document.write( "\n" ); document.write( "    - One characteristic property of arithmetic progressions\r
\n" ); document.write( "\n" ); document.write( "    - Solved 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
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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
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\n" ); document.write( "\n" ); document.write( "into your archive and use when it is needed.\r
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