document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=1143861>Question 1143861</A>: <I><SPAN STYLE=\"word-break: break-all;\"> The light bulb of a parabolic reflector is placed at the focus of the reflector for better reflection. Suppose that the reflector is 18 inches and 10 inches deep. What is the equation of the parabola in the parabolic reflector?<BR>\n" );
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document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #764805 by </B><A HREF=/tutors/aboutme.mpl?userid=Alan3354><B>Alan3354(69443)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=Alan3354><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=Alan3354><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=764805\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> The light bulb of a parabolic reflector is placed at the focus of the reflector for better reflection. Suppose that the reflector is 18 inches and 10 inches deep. What is the equation of the parabola in the parabolic reflector?<BR>\n" );
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document.write( "If you mean it's 18\" wide:<BR>\n" );
document.write( "If the parabola is symmetric about the y-axis and intersects the x-axis at (-9,0) and (9,0), then a 3rd point is (0,-10)<BR>\n" );
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document.write( "Use y = ax^2 + bx + c<BR>\n" );
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document.write( "For (-9,0):<BR>\n" );
document.write( "0 = 81a - 9b + c<BR>\n" );
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document.write( "For (9,0):<BR>\n" );
document.write( "0 = 81a + 9b + c<BR>\n" );
document.write( "0 = 81a - 9b + c<BR>\n" );
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document.write( "0 = 18b<BR>\n" );
document.write( "b = 0<BR>\n" );
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document.write( "For (0,-10)<BR>\n" );
document.write( "-10 = 0a + 0b + c<BR>\n" );
document.write( "c = -10<BR>\n" );
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document.write( "For (9,0):<BR>\n" );
document.write( "0 = 81a -10<BR>\n" );
document.write( "a = 10/81<BR>\n" );
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document.write( "A parabola is <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=10x%5E2%2F81+-+10 ALIGN=MIDDLE  ALT=\"10x%5E2%2F81+-+10\"   DISABLED_style= \"max-width:90%; height:auto;\" ><BR>\n" );
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document.write( "Note that this is A parabola, not THE parabola.  It can be moved and rotated which would give different equations.\r<BR>\n" );
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