Answer
Zero
Work Step by Step
1. If cotangent is undefined, it means that it has a value of zero in its denominator
$\cot\theta=\frac{a}{0}=undefined$
2. Since tangent is the reciprocal of cotangent, when cotangent is undefined the tangent is zero, independent from a.
$\tan\theta=\frac{0}{a}=0$
