chứng minh giùm mình lưỡng giác sau: -cos(3pi/2-2x)=-sin2x
[tex] - \cos \left( {\frac{{3\pi }}{2} - 2x} \right) = - \sin 2x[/tex]
[tex]VT = - \cos \left( {\frac{{3\pi }}{2} - 2x} \right) = - \cos \left( {2x - \frac{{3\pi }}{2}} \right)[/tex]
[tex]VT = - \sin \left( {2x - \pi } \right) = \sin \left( {\pi - 2x} \right) = \sin 2x \neq VP[/tex]
\Rightarrow PTLG [tex] - \cos \left( {\frac{{3\pi }}{2} - 2x} \right) = - \sin 2x[/tex]
[tex] \Leftrightarrow \cos \left( {2x - \frac{\pi }{2}} \right) = \sin \left( { - 2x} \right)[/tex]
[tex] \Leftrightarrow \sin 2x = \sin \left( { - 2x} \right)[/tex]
[tex] \Leftrightarrow 2x = - 2x + k2\pi \vee 2x = \pi - 2x + k2\pi [/tex]
[tex] \Leftrightarrow x = k\frac{\pi }{2} \vee x = (2k + 1)\frac{\pi }{4}[/tex]
[tex](k \in Z)[/tex]