Question 939614
sec^4 x -2sec^2x=tan^4 x-1
sec^2 x (sec^ x-2)=tan^4 x-1
tan^2 x+1(tan^2 x+1-2)=tan^4 x-1
tan^2 x+1(tan^2 x-1)=tan^4 x-1
tan^4 x-1=tan^4 x-1---------Q.E.D