$$
Now, we are given that none of $x_1, x_2,$ and $x_1 + x_2$ equals $(2k+1)\pi/2$ for any $k \in \mathbb{Z}$. This means that none of $\tan x_1, \tan x_2,$ and $\tan (x_1 + x_2)$ are undefined, since the tangent function is undefined at odd multiples of $\pi/2$.
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