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ΪʲôCosx+Cox3x=2Cos2x*Cosx

cos2x-x+cos2x+x =cos2xcosx+sin2xsinx+cos2xcosx-sin2xsinx =2cos2xcosx

cosxcos2xcos3xdx 1/2ңcosxcos3xcos3xdx1/2cosxcos3xdx1/2ңcos3x^2dx 1/4ңcos2xcos4xdx1/4ң1cos6xdx 1/4dx1/4cos2xdx1/4cos4xdx1/4cos6xdx 1/4x1...

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