$ \cos 3x.cos^{3}x - sin3x.sin^{3}x= \frac{2-3\sqrt{2} }{8} $
$ 9sinx + 6.cosx - 3sin2x + cos2x = 8 $
$ cos^{3}x + sin^{3}x = sinx - cosx $
câu 2) ; 3) đề đúng ko bạn??? –  ★·.·´¯`·.·★Poseidon★·.·´¯`·.·★ 12-07-16 08:17 PM
câu thứ 3 đề đúng ko bạn –  duongcscx 12-07-16 05:54 PM
$Pt\Leftrightarrow 9sinx+6cosx-6sinxcosx+1-2sin^2x=8$
$\Leftrightarrow -2sin^2x+9sinx-7-6cosx(sinx-1)=0$
$\Leftrightarrow (7-2sinx)(sinx-1)-6(sinx-1)=0$
$\Leftrightarrow (sinx-1)(1-2sinx)=0$

                                          Đến đây đơn giản rồi bạn tự giải nốt nha!!!
1) $Pt\Leftrightarrow cos3x.cos^3x+sin3x.sin^3x=\frac{2-3\sqrt{2}}{8}$
$\Leftrightarrow cos3x.cosx.cos^2x+sin3x.sinx.sin^2x=\frac{2-3\sqrt{2}}{8}$
$\Leftrightarrow \frac{1}{2}(cos4x+cos2x).cos^2x+\frac{1}{2}(cos2x-cos4x).sin^2x=...........$
$\Leftrightarrow \frac{1}{2}cos2x(sin^2x+cos^2x)+\frac{1}{2}cos4x.(cos^2x-sin^2x)=.......$
$\Leftrightarrow \frac{1}{2}cos2x+\frac{1}{2}cos4x.cos2x=......................$
$\Leftrightarrow cos2x+\frac{1}{2}(cos6x+cos2x)=\frac{2-3\sqrt{2}}{4}$
$3cos2x+cos6x=\frac{2-3\sqrt{2}}{2}$
Đến đây okey rồi nhé đưa về pt cơ bản dạng $asinx+bcosx=m$
a có thể giúp em mấy câu khác đk ko ạ??? –  kevotinhhn2k 19-07-16 08:33 PM
3) $Pt\Leftrightarrow (sinx+cosx)(sin^2x+cos^2x-sinxcosx)+cosx-sinx=0$
$\Leftrightarrow (cosx+sinx)(1-cosx.sinx)+cosx-sinx=0$
$\Leftrightarrow 2cosx-cosx.sinx.(cosx+sinx)=0$
$\Leftrightarrow cosx(2-sinx.cosx-sin^2x)=0$
$\Leftrightarrow cosx.(2-\frac{1}{2}sin2x-\frac{1-cos2x}{2})=0$ 
$\Leftrightarrow cosx.(\frac{3}{2}-\frac{1}{2}sin2x+\frac{1}{2}cos2x)=0$
$\Leftrightarrow cosx(3-sin2x+cos2x)=0$
Do$sin2x ; cos2x $ $\in [-1;1]$ $\Rightarrow 3-sin2x+cos2x>0$
$Pt\Rightarrow cosx=0\Leftrightarrow ....................$
 

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