[toán 11] bt C

H

huyentrang1996

Hi!!
Câu 1:
$\sqrt{3}sinx+cosx=\dfrac{1+\sqrt{2}}{2cosx}$
$\Leftrightarrow 2\sqrt{3}sinxcosx+2cos^2x=1+\sqrt{2}$
$\Leftrightarrow\sqrt{3}sin2x+cos2x=\sqrt{2}$
$\Leftrightarrow sin(2x+\dfrac{\pi}{6}+2x)=sin\dfrac{\pi}{4}$
Câu3
$sin^3x+4cos^3x=3sinx$
$\Leftrightarrow sin^3x+4cos^3x=3sinx(sin^2x+cos^2x)$
$\Leftrightarrow 2sin^3x+3sinxcos^2x-4cos^3x$(chia cho $cos^3x$ là được nhớ xét cosx=0)
 
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