Ủa, chào thanh niên, lâu ko gặp :)
\(\cos2\left(x-\frac{\pi}{4}\right)=1-2\sin^2\left(x-\frac{\pi}{4}\right)\Rightarrow\sin^2\left(x-\frac{\pi}{4}\right)=\frac{1-\cos\left(2x-\frac{\pi}{2}\right)}{2}\)
\(\cos2x=2\cos^2x-1\Leftrightarrow\cos^2x=\frac{\cos2x+1}{2}\)
\(\Rightarrow1-\cos\left(2x-\frac{\pi}{2}\right)=\cos2x+1\)
\(\Leftrightarrow\cos2x=\cos\left(2x-\frac{\pi}{2}+\pi\right)=\cos\left(2x+\frac{\pi}{2}\right)\)
\(\Leftrightarrow2x=\pm\left(2x+\frac{\pi}{2}\right)+k2\pi\)