document.write( "Question 3977: What is Sn of the geometric series with a1 = 4, an = 256 and n = 4? \n" ); document.write( "

Algebra.Com's Answer #1749 by longjonsilver(2297) $\"\"$ $\"About$

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$\"Sn+=+a%281-r%5En%29%2F%281-r%29\"$ \r
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\n" ); document.write( "\n" ); document.write( "need to find r. We know a=4 and that the fourth term, ar^3 = 256 --> r^3 = 64, so r = cube root of 64, so r=4.\r
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\n" ); document.write( "\n" ); document.write( "So, now: $\"Sn+=+4%281-4%5E4%29%2F%281-4%29\"$
\n" ); document.write( " $\"Sn+=+4%281-256%29%2F%28-3%29\"$
\n" ); document.write( "Sn = 340\r
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\n" ); document.write( "\n" ); document.write( "this is easy to check in this case, since the GP is small: 4+16+64+256 = 340.\r
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\n" ); document.write( "\n" ); document.write( "jon. \n" ); document.write( "

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