document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=792053>Question 792053</A>: <I><SPAN STYLE=\"word-break: break-all;\"> In an arithmetic sequence the 8th term is twice the 4th term and the 20th term is 40. Find the common difference and the sum of the terms from 8th to the 20th inclusive.. </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #479847 by </B><A HREF=/tutors/aboutme.mpl?userid=mananth><B>mananth(16946)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=mananth><IMG SRC=http://www.algebra.com/images/aboutme-small.gif BORDER=0 alt=\"About Me\" VALIGN=MIDDLE></A>&nbsp;<IMG SRC=http://www.algebra.com/cgi-bin/show-face.mpl?user=mananth><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=479847\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> In an arithmetic sequence the 8th term is twice the 4th term and the 20th term is 40. Find the common difference and the sum of the terms from 8th to the 20th inclusive..\r<BR>\n" );
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document.write( "a= first term<BR>\n" );
document.write( "n= number of terms<BR>\n" );
document.write( "d = common difference\r<BR>\n" );
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document.write( "8th. Term<BR>\n" );
document.write( "T8 = a+7d\r<BR>\n" );
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document.write( "T4 = a+3d\r<BR>\n" );
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document.write( "T8=2*T4\r<BR>\n" );
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document.write( "a+7d=2(a+3d)<BR>\n" );
document.write( "a+7d=2a+6d<BR>\n" );
document.write( "a-d=0<BR>\n" );
document.write( "a=d\r<BR>\n" );
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document.write( "T20 = a+19d\r<BR>\n" );
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document.write( "Therefore T4 = 4a<BR>\n" );
document.write( "T8=8a<BR>\n" );
document.write( "T20=20a<BR>\n" );
document.write( "40=20a<BR>\n" );
document.write( "a=2<BR>\n" );
document.write( "but a=d<BR>\n" );
document.write( "so d=2\r<BR>\n" );
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document.write( "S20= 10(4+19*2)\r<BR>\n" );
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document.write( "=420\r<BR>\n" );
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document.write( "S10= =5(4+18)<BR>\n" );
document.write( "S10 = 110\r<BR>\n" );
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document.write( "S20-S10= 420-110= 310\r<BR>\n" );
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document.write( " </SPAN>\n" );
document.write( "</TD></TR></TABLE>\n" );