Question

已知cos2a=3/5,求sina^4+cosa^4的值

Answer 1

sina^4+cosa^4=(sina^2+cosa^2)-2sina^2cosa^2

=1-2[1-cos2a)/2]*[(1+cos2a)/2]

=1-(1-cos^2a)/2

=1-[1-(3/5)^2]/2

=1-8/25=17/25