[toán 11]pt lượng giác.

N

ngoc1thu2

4sinx.cosx.cos2x.cos4x=-1

pt\Leftrightarrow 2sin2x.cos2x.cos4x=-1
\Leftrightarrowsin4x.cos4x=-1
\Leftrightarrowsin8x=-1/2
den day ban giai ra nhe:D

2cos^2(2x)+3cosx+1=0

pt\Leftrightarrow2.(cos^2(x)-1)^2+ 3cosx+1=0
\Leftrightarrow8cos^4(x)- 8cos^2(x)+ 3cosx +3 =0
\Leftrightarrow(cosx+1).(2cosx+1).(4cos^2(x)- 6cosx+3)=0
tiep tuc giai nha:D
 
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