cos(x)cos(2x)=[cosx+cos(3x)]/2
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¬å¼cosA+cosB=2cos[(A+B)/2] cos[(A-B)/2]
åçå¼å³è¾¹= [cosx+cos(3x)]/2
= [cos(3x)+cosx] /2
= cos[(3x+x)/2] cos[(3x-x)/2]
= 2cos(2x) cos(x)
= cos(x)cos(2x)
= åçå¼å·¦è¾¹
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