document.write( "Question 1131912: Write geometric series -9/2+3/2-1/2+1/6- ••• + 1/39366 in summation notation. Using the formula for the sum of an geometric series, compute the sum in the problem above. \n" ); document.write( "
Algebra.Com's Answer #748636 by MathLover1(20849)\"\" \"About 
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geometric series -9/2+3/2-1/2+1/6- ••• + 1/39366\r
\n" ); document.write( "\n" ); document.write( "\"a%5B1%5D\" -the first term
\n" ); document.write( "\"r\"-the \"common ratio\" between terms
\n" ); document.write( "\"a%5Bn%5D\" -the nth term\r
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\n" ); document.write( "\n" ); document.write( "\"a%5Bn%5D=a%5B1%5D%2Ar%5E%28n-1%29\"\r
\n" ); document.write( "\n" ); document.write( "\"a%5B1%5D=-9%2F2\" \r
\n" ); document.write( "\n" ); document.write( "\"r=%283%2F2%29%2F%28-9%2F2%29\"\r
\n" ); document.write( "\n" ); document.write( "\"r=%281%2F1%29%2F%28-3%2F1%29\"\r
\n" ); document.write( "\n" ); document.write( "\"r=%28-1%2F3%29\"\r
\n" ); document.write( "\n" ); document.write( "\"a%5Bn%5D=-%289%2F2%29%2A%28-1%2F3%29%5E%28n-1%29\"\r
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\n" ); document.write( "\n" ); document.write( "sum:\r
\n" ); document.write( "\n" ); document.write( "\"S%5Bn%5D=%28a%5B1%5D%281-r%5En%29%29%2F%281-r%29\"\r
\n" ); document.write( "
\n" ); document.write( "\n" ); document.write( "\"S%5Bn%5D=%28-%289%2F2%29%281-%28-1%2F3%29%5En%29%29%2F%281-%28-1%2F3%29%29\"...we need \"n\"\r
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\n" ); document.write( "\n" ); document.write( "use last given term \"1%2F39366\" and plug it in\r
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\n" ); document.write( "\n" ); document.write( "\"a%5Bn%5D=-%289%2F2%29%2A%28-1%2F3%29%5E%28n-1%29\"...last given term is \"1%2F39366\"\r
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\n" ); document.write( "\n" ); document.write( "\"1%2F39366=-%289%2F2%29%2A%28-1%2F3%29%5E%28n-1%29\"...solve for \"n\"\r
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\n" ); document.write( "\n" ); document.write( "\"%281%2F39366%29%2F%28-9%2F2%29=%28-1%2F3%29%5E%28n-1%29\"\r
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\n" ); document.write( "\n" ); document.write( "\"-1%2F177147=%28-1%2F3%29%5E%28n-1%29\"...take log of both sides\r
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\n" ); document.write( "\n" ); document.write( "\"-log%281%2F177147%29=-log%28%281%2F3%29%5E%28n-1%29%29\"\r
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\n" ); document.write( "\n" ); document.write( "\"-log%281%2F177147%29=-%28n-1%29log%28%281%2F3%29%29\"\r
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\n" ); document.write( "\n" ); document.write( "\"n-1=%28-log%281%2F177147%29%29%2F%28-log%28%281%2F3%29%29%29\"\r
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\n" ); document.write( "\n" ); document.write( "\"n-1=log%281%2F177147%29%2Flog%28%281%2F3%29%29\"\r
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\n" ); document.write( "\n" ); document.write( "\"n-1=11\"\r
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\n" ); document.write( "\n" ); document.write( "\"n=11%2B1\"\r
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\n" ); document.write( "\n" ); document.write( "\"n=12\"\r
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\n" ); document.write( "\n" ); document.write( "=>\r
\n" ); document.write( "\n" ); document.write( "\"S%5B12%5D=%28-%289%2F2%29%281-%28-1%2F3%29%5E12%29%29%2F%281%2B1%2F3%29\"\r
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\n" ); document.write( "\n" ); document.write( "\"S%5B12%5D=%28-%289%2F2%29%281-1%2F531441%29%29%2F%284%2F3%29\"\r
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\n" ); document.write( "\n" ); document.write( "\"S%5B12%5D=%28-%289%2F2%29%28531440%2F531441%29%29%2F%284%2F3%29\"\r
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\n" ); document.write( "\n" ); document.write( "\"S%5B12%5D=%28-265720%2F59049%29%2F%284%2F3%29\"\r
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\n" ); document.write( "\n" ); document.write( "\"S%5B12%5D=-66430%2F19683\"\r
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\n" ); document.write( "\n" ); document.write( "\"S%5B12%5D=-3\"\"+7381%2F19683\"\r
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