document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=1005071>Question 1005071</A>: <I><SPAN STYLE=\"word-break: break-all;\"> Using Trigonometry Identities. Simplify the expression into one trigonometric function then evaluate if possible. \r<BR>\n" );
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document.write( "Cos 13pi/18 * Cos pi/18 + Sin 13pi/18 * Sin pi/18\r<BR>\n" );
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document.write( "I tried this problem many different ways but I'm not sure exactly how to do it. </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #621375 by </B><A HREF=/tutors/aboutme.mpl?userid=ikleyn><B>ikleyn(52781)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=ikleyn><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=ikleyn><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=621375\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> .<BR>\n" );
document.write( "Using Trigonometry Identities. Simplify the expression into one trigonometric function then evaluate if possible. \r<BR>\n" );
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document.write( "Cos 13pi/18 * Cos pi/18 + Sin 13pi/18 * Sin pi/18<BR>\n" );
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document.write( "It is simple. &nbsp;The key is to apply the subtraction formula for cosines:\r<BR>\n" );
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document.write( "Cos(<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=13pi%2F18 ALIGN=MIDDLE  ALT=\"13pi%2F18\"   DISABLED_style= \"max-width:90%; height:auto;\" >) * Cos(<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=pi%2F18 ALIGN=MIDDLE  ALT=\"pi%2F18\"   DISABLED_style= \"max-width:90%; height:auto;\" >) + Sin(<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=13pi%2F18 ALIGN=MIDDLE  ALT=\"13pi%2F18\"   DISABLED_style= \"max-width:90%; height:auto;\" >) * Sin(<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=pi%2F18 ALIGN=MIDDLE  ALT=\"pi%2F18\"   DISABLED_style= \"max-width:90%; height:auto;\" >) =\r<BR>\n" );
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document.write( "&nbsp;&nbsp;&nbsp;&nbsp;( use the facts that Cos(<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=pi%2F18 ALIGN=MIDDLE  ALT=\"pi%2F18\"   DISABLED_style= \"max-width:90%; height:auto;\" >) = Cos(<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=-pi%2F18 ALIGN=MIDDLE  ALT=\"-pi%2F18\"   DISABLED_style= \"max-width:90%; height:auto;\" >), Sin(<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=pi%2F18 ALIGN=MIDDLE  ALT=\"pi%2F18\"   DISABLED_style= \"max-width:90%; height:auto;\" >) = -Sin(<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=-pi%2F18 ALIGN=MIDDLE  ALT=\"-pi%2F18\"   DISABLED_style= \"max-width:90%; height:auto;\" >) )\r<BR>\n" );
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document.write( "= Cos(<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=13pi%2F18 ALIGN=MIDDLE  ALT=\"13pi%2F18\"   DISABLED_style= \"max-width:90%; height:auto;\" >) * Cos(<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=-pi%2F18 ALIGN=MIDDLE  ALT=\"-pi%2F18\"   DISABLED_style= \"max-width:90%; height:auto;\" >) - Sin(<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=13pi%2F18 ALIGN=MIDDLE  ALT=\"13pi%2F18\"   DISABLED_style= \"max-width:90%; height:auto;\" >) * Sin(<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=-pi%2F18 ALIGN=MIDDLE  ALT=\"-pi%2F18\"   DISABLED_style= \"max-width:90%; height:auto;\" >) = \r<BR>\n" );
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document.write( "&nbsp;&nbsp;&nbsp;&nbsp;(now apply the subtraction formula for cosines, &nbsp;see the lesson \r<BR>\n" );
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document.write( "&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<A  rel=nofollow HREF=http://www.algebra.com/algebra/homework/Trigonometry-basics/Addition-and-subtraction-formulas.lesson>Addition and subtraction formulas</A>&nbsp; in this site) \r<BR>\n" );
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document.write( "= cos (<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=13pi%2F18 ALIGN=MIDDLE  ALT=\"13pi%2F18\"   DISABLED_style= \"max-width:90%; height:auto;\" > - <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=pi%2F18 ALIGN=MIDDLE  ALT=\"pi%2F18\"   DISABLED_style= \"max-width:90%; height:auto;\" >) = cos(<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=12pi%2F18 ALIGN=MIDDLE  ALT=\"12pi%2F18\"   DISABLED_style= \"max-width:90%; height:auto;\" >) = cos(<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=2pi%2F3 ALIGN=MIDDLE  ALT=\"2pi%2F3\"   DISABLED_style= \"max-width:90%; height:auto;\" >) = <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=-1%2F2 ALIGN=MIDDLE  ALT=\"-1%2F2\"   DISABLED_style= \"max-width:90%; height:auto;\" >. \r<BR>\n" );
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document.write( "That's all.\r<BR>\n" );
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document.write( " </SPAN>\n" );
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