document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=710571>Question 710571</A>: <I><SPAN STYLE=\"word-break: break-all;\"> prove that cosAcos(60-A)cos(60+A)=1/4cos3A. </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #437046 by </B><A HREF=/tutors/aboutme.mpl?userid=srinivas.g><B>srinivas.g(540)</B></A><img src=\"/images/stars/stars-10.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=srinivas.g><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=srinivas.g><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=437046\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\">   formula for cox( x+y) cos(x-y) = cos ^2 (y) - sin^2( x)\r<BR>\n" );
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document.write( " here cos (60-A) cos (60+A) = cos^2(A)-  sin^2 (60)<BR>\n" );
document.write( "                            = cos ^2 (A)  - 3/4<BR>\n" );
document.write( "        now cos A cos (60-A) cos (60+A  = cos A ( cos^2 (A) - 3/4)<BR>\n" );
document.write( "\                                        = 1/4 (  4 cos ^3 (a) - 3 cos A)<BR>\n" );
document.write( "{since  4 cos ^3 (a) - 3 cos A = cos 3A  }<BR>\n" );
document.write( "                                         = 1/4 ( cos 3A)<BR>\n" );
document.write( "hence proved\r<BR>\n" );
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document.write( "  </SPAN>\n" );
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