document.write( "Question 1005071: Using Trigonometry Identities. Simplify the expression into one trigonometric function then evaluate if possible. \r
\n" ); document.write( "\n" ); document.write( "Cos 13pi/18 * Cos pi/18 + Sin 13pi/18 * Sin pi/18\r
\n" ); document.write( "\n" ); document.write( "I tried this problem many different ways but I'm not sure exactly how to do it. \n" ); document.write( "
Algebra.Com's Answer #621384 by MathTherapy(10552)\"\" \"About 
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Using Trigonometry Identities. Simplify the expression into one trigonometric function then evaluate if possible. \r
\n" ); document.write( "\n" ); document.write( "Cos 13pi/18 * Cos pi/18 + Sin 13pi/18 * Sin pi/18\r
\n" ); document.write( "\n" ); document.write( "I tried this problem many different ways but I'm not sure exactly how to do it.
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You need to use the \"Difference of 2 angles\" Identity, not the \"Sum of 2 angles\" identity.
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document.write( "Difference of 2 angles identity: cos(A - B) = cos A cos B + sin A sin B. Compare this to: \"cos+%2813pi%2F18%29+%2A+cos+%28pi%2F18%29+%2B+sin+%2813pi%2F18%29+%2A+sin+%28pi%2F18%29\"
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document.write( "\"cos%28A+-+B%29+=+cos+%2813pi%2F18+-+pi%2F18%29\" = \"cos+%2812pi%2F18%29\"
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document.write( "Reducing \"12pi%2F18\", we get: \"2pi%2F3%29\"
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document.write( "\"cos+%282pi%2F3%29\" is in the 2nd quadrant, its reference angle is: \"pi%2F3\" and it's negative (< 0), so \"cos+%282pi%2F3%29\" = \"-+cos+%28pi%2F3%29\" = \"highlight_green%28-+1%2F2%29\" \n" );
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