PTLG có dấu giá trị tuyệt đối.11

K

kenny_111

ko ai chem het de minh chem thu?
1) cos2x= |cosx|+|sinx|
ban xet lan luot cac truong hop c0sx=0 sinx=0 cos2x=0 roi lap bang xet dau trong khoang [0;2pi]
roi chia ra cac khoang bien luan ( giong nam lop 10 ay' dai qua nen minh hoi luoi :p )
2)
|cosx+2sin2x-cos3x|=1+2sinx-cos2x
| 2sinx +(cosx-cos3x)|= 1+2sinx-(1-2sin^2x)
|2sinx +2sin2xsinx|=2sin(sinx+1)
| 2sinx(1+sin2x| =2sinx(sinx+1)
|2sinx|(1+sin2x)=2sinx(sinx+1)
den day ban chia ra 2 truong hop
sinx>=0 vs sinx<0 de giai
3)
|cos3x|=1-can3sinx
1-can3sinx>=0
{
(cos3x)^2=(1-can3sinx)^2
 
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