Đáp án:

$\text{$C$ = $cos4x$ + 4$cos2x$ + 3}$ $\\$ $\text{$C$ = $2cos^22x$ $-$ 1 + 4$cos2x$ + 3}$ $\\$ $\text{$C$ = $2cos^22x$ + 4$cos2x$ + 2}$ $\\$ $\text{$C$ = 2($cos^22x$ + 2$cos2x$ + 1)}$ $\\$ $\text{$C$ = 2$(cos2x+1)^2$}$ $\\$ $\text{$C$ = 2$(2cos^2x-1+1)^2$}$ $\\$ $\text{$C$ = 2$(2cos^2x)^2$}$ $\\$ $\text{$C$ = 8$cos^4x$}$

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