The trigonometric identity `(tan^2 x) / (1 + tan^2 x) = tan x` has to be proved.

`(tan^2 x) / (1 + tan^2 x)`

Use the fact that `tan x = sin x/cos x`

=> `((sin^2x)/(cos^2x))/(1 + (sin^2x)/(cos^2x))`

=> `((sin^2x)/(cos^2x))/((cos^2x + sin^2x)/(cos^2x))`

`cos^2x + sin^2x = 1`

=> `((sin^2x)/(cos^2x))/(1/(cos^2x))`

=>` sin^2x`

As shown above `(tan^2 x) / (1 + tan^2 x) = sin^2x` not tan x.

**The given relation is not an identity. It holds for only a few angles not as a general rule.**