Answer: The required answer is tan x.
Step-by-step explanation: We are given to complete the following trigonometric identity :

We will be using the following identity from trigonometry to complete the given identity :

Now, we have
![(\sec x-1)(\sec x+1)\\\\=\sec^2x-1\\\\=\tan^2x~~~~~~~~~~[\textup{from equation (i)}]\\\\=(\tan x)^2.](https://img.qammunity.org/2020/formulas/mathematics/middle-school/4nnga3vp0u7nw1xiiqviref59ygeyfzwep.png)
Thus, the complete identity is

Thus, the required answer is tan x.