document.write( "Question 976584: Simplify cos(x)cos(y) + sin(x)sin(y) \n" ); document.write( "

Algebra.Com's Answer #598142 by Edwin McCravy(20059) $\"\"$ $\"About$

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Simplify cos(x)cos(y) + sin(x)sin(y)
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document.write( "Such problems as this only require that you have memorized certain \r\n" );
document.write( "standard double-angle identities.\r\n" );
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document.write( "If you had memorized the identity\r\n" );
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document.write( "cos(A-B) = cos(A)cos(B) + sin(A)sin(B)\r\n" );
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document.write( "The you would have been able to recognize\r\n" );
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document.write( "cos(x)cos(y) + sin(x)sin(y)\r\n" );
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document.write( "as the right side of that identity with x substituted for A, and y\r\n" );
document.write( "substituted for B, and you'd just make those same substitutions\r\n" );
document.write( "in the left side of the identity as well and get:\r\n" );
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document.write( "cos(x-y) = cos(x)cos(y) + sin(x)sin(y)\r\n" );
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document.write( "Be sure to memorize the double-angle identities for sine, cosine and tangent\r\n" );
document.write( "so you'll be able to do such problems on tests.\r\n" );
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document.write( "Edwin

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