Tìm m để pt có nhiều hơn 1 nhiệm x thuộc

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niemkieuloveahbu

[TEX]2cosxcos2xcos3x+m=7cos2x\\ \Leftrightarrow cos2x(cos2x+cos4x)+m=7cos2x\\ \Leftrightarrow 2cos^32x+cos^22x-8cos2x=-m\\ Dat\ cos2x=t, x \in [-\frac{3\pi}{8};\frac{-\pi}{8}] \Rightarrow t \in [-\frac{\sqrt{2}}{2}, \frac{\sqrt{2}}{2}], moi\ nghiem\ t\ chi\ cho\ 1\ nghiem\ x\\ PT\ tro\ thanh:\\ 2t^3+t^2-8t=-m(1)\\ DK\ bai\ tro\ thanh\ tim\ m\ de\ (1) \ co\ nhieu\ hon\ 1\ nghiem.\\ y'=6t^2+2t-8=0\Leftrightarrow \left[x=1\\x=-\frac{4}{3}\right.\\ Lap\ BBT\ \Rightarrow kq. [/TEX]

^^
 
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