Explicație pas cu pas:
Folosim formula fundamentala a trigonometriei:
[tex]\sin^2x+\cos^2x=1\\\sin^2x=1-\cos^2x[/tex]
Ridicam la patrat si avem:
[tex]\sin^2x=1-\cos^2x|^2\\(\sin^2x)^2=(1-\cos^2x)^2\\\sin^4x=1+\cos^4x-2\cos^2x[/tex]
Si atunci membrul stang devine:
[tex]\sin^4x-\sin^2x=1+\cos^4x-2\cos^2x-(1-\cos^2x)=1+\cos^4x-2\cos^2x-1+\cos^2x=\cos^4x-\cos^2x[/tex]
