lượng giác 10

B

buivanbao123

a)Ta có:VT=$\dfrac{tanx}{sinX}-\dfrac{sinx}{cotx}$

=$\dfrac{1}{cosx}-\dfrac{sin^{2}x}{cosx}$

=$\frac{1-sin^{2}x}{cosx}$

=$\dfrac{cos^{2}x}{cosx}$=cosx(đpcm)

b)A=$cos^{4}+sin^{2}.cos^{2}+cos^{4}-sin^{4}+3sin^{2}x$

\Leftrightarrow A=$cos^{2}x(cos^{2}x+sin^{2}x)+cos^{2}x-sin^{2}x+3sin^{2}x$

\Leftrightarrow A=$2cos^{2}x+2sin^{2}x=2$
 
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