có bài hạ bậc cần mọi người giúp đỡ..

M

metla2011

pt[TEX]\Leftrightarrow 8\sqrt{2}cos^6x-6\sqrt{2}cos^4x+2\sqrt{2}sin^3x(3sinx-4sin^3x)-1=0[/TEX]
[TEX]\Leftrightarrow 8\sqrt{2}(cos^6x-sin^6x)+6\sqrt{2}(sin^4x-cos^4x)-1=0[/TEX]
[TEX]\Leftrightarrow 8\sqrt{2}[cos^3 2x+\frac{3}{4}(1-cos^2 2x)cos2x]-6\sqrt{2}cos2x-1=0[/TEX]
[TEX]\Leftrightarrow 2\sqrt{2}cos^3 2x=1[/TEX]
[TEX]\Leftrightarrow cos2x=\sqrt[3]{\frac{\sqrt{2}}{4}}[/TEX]
[TEX]\Rightarrow x=\frac{pi}{8}+kpi;x=-\frac{pi}{8}+kpi[/TEX]
 
You must log in or register to reply here.
Top Bottom