You can put this solution on YOUR website! prove the idenity true
sec^4x-(tan^4x+sec^2x)=sec^2x*sin^2x
starting with left side
sec^4x-(tan^4x+sec^2x)
1/cos^4-sin^4/cos^4-1/cos^2
LCD:cos^4
(1-sin^4-cos^2)/cos^4
(sin^2-sin^4)/cos^4
sin^2(1-sin^2)/cos^4
sin^2*cos^2/cos^4
sin^2/cos^2
sin^2*sec^2
left side=right side