document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=836583>Question 836583</A>: <I><SPAN STYLE=\"word-break: break-all;\"> Hello, I don't know if this is a linear equation or not, sorry.\r<BR>\n" );
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document.write( "My question is: I need a formula to calculate the total cost of something. But the more I have, the more it costs.\r<BR>\n" );
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document.write( "Example:<BR>\n" );
document.write( "Price per unit: $10<BR>\n" );
document.write( "Price for second unit: $15<BR>\n" );
document.write( "Price for third unit: $20<BR>\n" );
document.write( "So for every extra unit I have, it will add $5.\r<BR>\n" );
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document.write( "Formula/equation I need is:<BR>\n" );
document.write( "Find Total cost. I have 5 units on hand ($100). I need to purchase another 10 units. So it should add $105 (6th unit) + $110 + $115 ... + $145 (9th unit) + $150 (10th unit).<BR>\n" );
document.write( "Would be nice to have a formula where I can plug in the numbers. For example, I have 5 units on hand so my next starting cost would be $105, and I will buy 7 more units, then my total price would be x.\r<BR>\n" );
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document.write( "Thank you very much for your time. </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #504271 by </B><A HREF=/tutors/aboutme.mpl?userid=richard1234><B>richard1234(7193)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=richard1234><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=richard1234><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=504271\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> The n-th unit costs $5n+5. Also, the cost for 5 units is 10+15+...+30 = $100, but be careful; the sixth unit costs $35, not $105.\r<BR>\n" );
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document.write( "For n >= 1, the cost for n units is\r<BR>\n" );
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document.write( "<IMG STYLE=\"max-width:80%;\" SRC=\"/cgi-bin/rendertex.cgi?\Large 10 + 15 + \ldots + 5n+5\">\r<BR>\n" );
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document.write( "This is one of those \"Gauss sums,\" and the sum is <IMG STYLE=\"max-width:80%;\" SRC=\"/cgi-bin/rendertex.cgi?\Large \frac{(5n+15)n}{2} = \frac{5n(n+3)}{2}\">. </SPAN>\n" );
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