document.write( "Question 1206130: a theater has 20 rows of chairs the first row has 10 chairs and each row after that has 2 more seats than the row before it
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Algebra.Com's Answer #843374 by ikleyn(52795)\"\" \"About 
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\n" ); document.write( "a theater has 20 rows of chairs the first row has 10 chairs and each row after that
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document.write( "The series of the chairs in rows is an arithmetic progression.\r\n" );
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document.write( "1-st row has 10 chairs.\r\n" );
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document.write( "The last, 20th row has 10 + (20-1)*2 = 10 + 19*2 = 10 + 38 = 48 chairs.\r\n" );
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document.write( "(use the formula for the n-th term of an AP).\r\n" );
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document.write( "In all, there are  \"%28%28a%5B1%5D%2Ba%5B20%5D%29%2F2%29%2A20\" = \"%28%2810%2B48%29%2F2%29%2A20\" = \"%2858%2F2%29%2A20\" = 58*10 = 580 chairs.\r\n" );
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document.write( "(use the formula for the sum of the first n terms of an AP).\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
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\n" ); document.write( "\n" ); document.write( "    - Word problems on arithmetic progressions\r
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