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ΪʲôCosx+Cox3x=2Cos2x*Cosx

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

lim (cosx-cos3x)/x² x0 =lim 2sin2xsinx/x² x0 =lim 22sinxcosxsinx/x² x0 =lim 4x²cosx/x² x0 =lim 4cosx x0 =4cos0 =41 =4

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