Ta có:
\(\cos2\alpha\) = \(\cos^2\alpha\)- \(\sin^2\alpha\)=\(\dfrac{3}{5}\)
⇒ \(\sin^4\alpha\)- \(\cos^4\alpha\)= -(\(\sin^2\alpha\)+ \(\cos^2\alpha\))(\(\cos^2\alpha\)-\(\sin^2\alpha\)) = -1.\(\dfrac{3}{5}\)= -\(\dfrac{3}{5}\)
VẬY ...............