document.write( "Question 1036737: In an arithmetic progression, the thirteenth term
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Algebra.Com's Answer #651541 by ikleyn(52781)\"\" \"About 
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\n" ); document.write( "In an arithmetic progression, the thirteenth term
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document.write( "We are given \r\n" );
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document.write( "\"a%5B13%5D\" = 27,         (1)\r\n" );
document.write( "\"a%5B7%5D\"  = \"3%2Aa%5B2%5D\".      (2)\r\n" );
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document.write( "which, in other terms, is\r\n" );
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document.write( "\"a%5B1%5D\" + 12*d = 27,           (1')\r\n" );
document.write( "\"a%5B1%5D\" +  6*d = \"3%2A%28a%5B1%5D%2Bd%29\".   (2').\r\n" );
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document.write( "Simplify it and write in the canonical form for the system of two equations in two unknowns:\r\n" );
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document.write( "  \"a%5B1%5D\" + 12*d = 27,         (1'')\r\n" );
document.write( "\"-2a%5B1%5D\" +  3*d =  0.         (2'')\r\n" );
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document.write( "Now, based on (2''), replace the term 12*d in (1'') by \"8a%5B1%5D\". You will get a single equation for \"a%5B1%5D\"\r\n" );
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document.write( "\"a%5B1%5D+%2B+8a%5B1%5D\" = 27,   or   \"9%2Aa%5B1%5D\" = 27,  which gives you  \"a%5B1%5D\" = 3.\r\n" );
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document.write( "Then from (2'')  3d = \"2a%5B1%5D\" = 6.  Hence,  d = 2.\r\n" );
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document.write( "Answer.  \"a%5B1%5D\" = 3,  d = 2.\r\n" );
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