giúp mình bài này nhé

`A = (1-tan^2x)^2/(4tan^2x)-1/(4sin^2xcos^2x)`

`=((1-tan^2x)^2*sin^2xcos^2x-tan^2x)/(4tan^2xsin^2xcos^2x)`

`=(((cos^2x-sin^2x)/(cos^2x))^2*sin^2xcos^2x-tan^2x)/(4sin^4x)`

`=((1-4sin^2xcos^2x)/(cos^4x)*sin^2xcos^2x-tan^2x)/(4sin^4x)`

`=((1-4sin^2x+tan^2x)*sin^2x-tan^2x)/(4sin^4x)`

`=(sin^2x-4sin^4x+tan^2x(sin^2x-1))/(4sin^4x)`

`=(sin^2x-4sin^4x-sin^2x)/(4sin^4x)`

`=-1`