Step-by-step explanation:
[tex]\sin^{4}x \: - \cos^{4} x = (\sin^{2}x)^{2} - (\cos^{2}x)^{2} [/tex]
[tex] = (\sin^{2}x)^{2} - (1 - \sin^{2}x)^{2} [/tex]
[tex] = \sin^{4}x - (1 - 2\sin^{2}x + \sin^{4}x)[/tex]
[tex] = 2\sin^{2}x - 1[/tex]
[tex] = - \cos2x[/tex]
Note: I used the identity
[tex] { \sin }^{2} x = \frac{1}{2} (1 - \cos \: 2x)[/tex]
for the last step.
PS. I love proving trigonometric identities!
