document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=856229>Question 856229</A>: <I><SPAN STYLE=\"word-break: break-all;\"> The sum of the first 7 terms of an A.P is 28 and the sum of the first 15 terms is 90. Find the sum of the n term </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #515804 by </B><A HREF=/tutors/aboutme.mpl?userid=richwmiller><B>richwmiller(17219)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=richwmiller><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=richwmiller><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=515804\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> S = ½(a + an)n<BR>\n" );
document.write( "S = ½(2a + (n-1)d)n<BR>\n" );
document.write( "an = a + (n - 1)d\r<BR>\n" );
document.write( "\n" );
document.write( "S = ½(2a + (n-1)d)n<BR>\n" );
document.write( "28= ½(2a + 6d)7<BR>\n" );
document.write( "90 = 1/2(2a+ 14d)15<BR>\n" );
document.write( "28=(a + 3d)7,<BR>\n" );
document.write( "90 = (a+ 7d)15<BR>\n" );
document.write( "a = 5/2,   d = 1/2<BR>\n" );
document.write( "S = ½(5/2 + An)n<BR>\n" );
document.write( "or<BR>\n" );
document.write( "S = ½(2*5/2 + 1/2(n-1))n<BR>\n" );
document.write( "S = 1/4 n(n+9) </SPAN>\n" );
document.write( "</TD></TR></TABLE>\n" );