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2)Sin^3 x+Cos^3x=Cos2x(hay là =Sin2x vậy các bạn???)
\Leftrightarrow [TEX]cos^3x - cos^2x + sin(1-cos^2x) =0[/TEX]
\Leftrightarrow [TEX]cos^3x - cos^2x + sinx - sinxcos^2x = 0[/TEX]
\Leftrightarrow [TEX] -cos^2x(1-cosx) + sinx(1-cos^2x) = 0[/TEX]
\Leftrightarrow[TEX](1-cosx)(-cos^2x + sinx(1+cosx) = 0[/TEX]
\Leftrightarrow[TEX](1-cosx)(sinx + sinxcosx - cos^2x) = 0[/TEX]
\Leftrightarrow [TEX]1 - cosx = 0[/TEX] \Rightarrow.........
[TEX]sinx + sinxcosx - cos^2x = 0[/TEX]
\Leftrightarrow[TEX]sinx + sinxcosx + sin^2x - 1 = 0[/TEX]
\Leftrightarrow[TEX]sinx(1+cosx + sinx) =1[/TEX]
\Rightarrow sinx = 1 và 1+ cosx + sinx = 1 \Rightarrow...........
[TEX]1)Sin^4 x + Cos^4x = \frac{3 - Cos6x}{4}[/TEX]
[TEX]\leftrightarrow (sin^2x + cos^2x)^2 - 2sin^2x.cos^2x = \frac{3 - Cos6x}{4} [/TEX]
[TEX]\leftrightarrow 1 - \frac{1}{2}sin^22x = \frac{3 - Cos6x}{4}[/TEX]
[TEX]\leftrightarrow 4 - 2sin^22x = 3 - 4cos6x[/TEX]
[TEX]\leftrightarrow 4cos6x - 2sin^22x + 1 = 0[/TEX]
[TEX]\leftrightarrow 4.(4cos^32x - 3cos2x) - 2sin^22x + 1 = 0[/TEX]
[TEX]\leftrightarrow 16sin^32x - 2sin^22x - 12cos2x + 1 = 0[/TEX]
[TẶNG BẠN] TRỌN BỘ Bí kíp học tốt 08 môn