[Toán 11] Phương trinhg lượng giác

N

niemkieuloveahbu

[TEX]PT\Leftrightarrow2{sin}^{2}x.cosx.cos5x=1 [/TEX] (*)
[TEX]Do\ cos5x \leq cosx[/TEX]
[TEX]\Rightarrow (*) \leq 2{sin}^{2}x.{cos}^{2}x = \frac{1}{2}{sin}^{2}2x[/TEX]
[TEX]\Rightarrow 1 \leq \frac{1}{2}{sin}^{2}2x\Rightarrow 2 \leq {sin}^{2}2x[/TEX] ( vô lý)
PT vô nghiệm
 
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