document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=207289>Question 207289</A>: <I><SPAN STYLE=\"word-break: break-all;\"> Consider an area of  cm2 bounded by two concentric circles ( the figure is called an annulus). Determine the radius of the inner circle ( in units of cm) if the outer and the inner diameters of the of the annulus differ by 2 cm. </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #156770 by </B><A HREF=/tutors/aboutme.mpl?userid=ankor@dixie-net.com><B>ankor@dixie-net.com(22740)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=ankor@dixie-net.com><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=ankor@dixie-net.com><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=156770\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> : Consider an area of cm2 bounded by two concentric circles ( the figure is called an annulus). Determine the radius of the inner circle ( in units of cm) if the outer and the inner diameters of the of the annulus differ by 2 <BR>\n" );
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document.write( "The area of an annulus formula<BR>\n" );
document.write( "A = <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=pi%28R%5E2+-+r%5E2%29 ALIGN=MIDDLE  ALT=\"pi%28R%5E2+-+r%5E2%29\"   DISABLED_style= \"max-width:90%; height:auto;\" ><BR>\n" );
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document.write( "Given that R - r = 2; therefore:<BR>\n" );
document.write( "R = (r+2)<BR>\n" );
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document.write( "Substitute (r+2) for r in the annulus formula:<BR>\n" );
document.write( "A = <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=pi%28%28r%2B2%29%5E2+-+r%5E2%29 ALIGN=MIDDLE  ALT=\"pi%28%28r%2B2%29%5E2+-+r%5E2%29\"   DISABLED_style= \"max-width:90%; height:auto;\" ><BR>\n" );
document.write( "FOIL<BR>\n" );
document.write( "A = A = <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=pi%28r%5E2+%2B+4r+%2B+4+-+r%5E2%29 ALIGN=MIDDLE  ALT=\"pi%28r%5E2+%2B+4r+%2B+4+-+r%5E2%29\"   DISABLED_style= \"max-width:90%; height:auto;\" ><BR>\n" );
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document.write( "A = <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=pi%284r%2B4%29 ALIGN=MIDDLE  ALT=\"pi%284r%2B4%29\"   DISABLED_style= \"max-width:90%; height:auto;\" ><BR>\n" );
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document.write( "A = <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=4pi%28r%2B1%29 ALIGN=MIDDLE  ALT=\"4pi%28r%2B1%29\"   DISABLED_style= \"max-width:90%; height:auto;\" ><BR>\n" );
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document.write( "r + 1 = <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=A%2F%284pi%29 ALIGN=MIDDLE  ALT=\"A%2F%284pi%29\"   DISABLED_style= \"max-width:90%; height:auto;\" ><BR>\n" );
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document.write( "r = <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=A%2F%284pi%29 ALIGN=MIDDLE  ALT=\"A%2F%284pi%29\"   DISABLED_style= \"max-width:90%; height:auto;\" > - 1<BR>\n" );
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document.write( "Just substitute the value for A to find r (smaller radius)<BR>\n" );
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