document.write( "Question 1142055: Show that
\n" ); document.write( "2sin 4pi/5 cos pi/5 = sin 2pi/5 \n" ); document.write( "

Algebra.Com's Answer #762712 by ikleyn(53756) $\"\"$ $\"About$

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document.write( "First use the identity  \r\n" );
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document.write( "     = ,\r\n" );
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document.write( "which is valid for any angle .  From this identity, you get\r\n" );
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document.write( "     = .\r\n" );
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document.write( "It gives you\r\n" );
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document.write( "     = .\r\n" );
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document.write( "Next use the identity   = ,  which is valid for any angle  .  It gives you\r\n" );
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document.write( "     =  = .\r\n" );
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document.write( "It is what has to be proved.\r\n" );
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