sina=√5/5 所以：sin^2a=1/5
又因：sin^2a+cos^2a=1 所以：cos^2a=4/5
sin^4a-cos^4a
=(sin^2a+cos^2a)(sin^2a-cos^2a)
=sin^2a-cos^2a
=1/5-4/5
=-3/5那个是sina的四次方我做的也是sina的四次方不客气