cos4α/(cos2α-sin2α)=cos4α*(cos2α+sin2α)/((cos2α-sin2α)(cos2α+sin2α)=
=cos4α*(sin2α+cos2α)/(cos²2α-sin²2α)=cos4α*(sin2α+cos2α)/cos4α=
=sin2α+cos2α.
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cos4α/(cos2α-sin2α)=cos4α*(cos2α+sin2α)/((cos2α-sin2α)(cos2α+sin2α)=
=cos4α*(sin2α+cos2α)/(cos²2α-sin²2α)=cos4α*(sin2α+cos2α)/cos4α=
=sin2α+cos2α.