[tex]\frac{\sin x \cos x}{\sin^4 x+\cos^4 x}=\frac{\sin x \cos x}{\sin^4 x+\cos^4 x+2\sin^2 x\cos^2 x-2\sin^2 x\cos^2 x} =[/tex]
[tex]= \frac{\sin x \cos x}{(\sin^2 x+\cos^2 x)^2-2\sin^2 x\cos^2 x}=\frac{\sin x \cos x}{1-2\sin^2 x\cos^2 x}=[/tex]
[tex]=\frac{2\sin x \cos x}{2-4\sin^2 x\cos^2 x}=\frac{\sin 2x}{2-(2\sin x\cos x)^2}=\frac{\sin 2x}{2-\sin^2 2x}=\frac{\sin 2x}{1+\cos^2 2x}[/tex]
Нататък мисля, че знаеш какво да правиш

.