$y=sin^4x+2cos^4x$

E

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Xem lại PT:

$sin^4x \ge 0$

$2cos^4x \ge 0$

$sin^4x+2cos^4x=0$

\Leftrightarrow $sin^4x=0;cos^4x=0$

\Leftrightarrow $sinx=0;cosx=0$ (vn)
 
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