document.write( "Question 999847: the last three terms of an arithmetic sequence with 18 terms are 67, 72,77 . Find the first term and the sum of the series \n" ); document.write( "

Algebra.Com's Answer #617394 by AnlytcPhil(1806) $\"\"$ $\"About$

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document.write( "The other tutor forgot the sum.\r\n" );
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document.write( "The hard way is to notice that 77-72 = 5 and 72-67 = 5, and then list\r\n" );
document.write( "them from right to left like this, subtracting 5 each time \r\n" );
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document.write( "-8 + -3 + 2 + 7 + 12 + 17 + 22 + 27 + 32 + 37 + 42 + 47 + 52 + 57 + 62 + 67 + 72 + 77\r\n" );
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document.write( "And get -8 for the first term\r\n" );
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document.write( "And then add them up, getting 621.\r\n" );
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document.write( "The easy way is to use the formula\r\n" );
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document.write( "where , , \r\n" );
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document.write( "and the sum formula:\r\n" );
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document.write( "Edwin

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