I don’t understand this please help

The cotangent function is the inverse of the tangent function:

The domain of a function is the set of points where you can evaluate the function. Since the cotangent function is a ratio of cosine and sine, if the sine equals zero we can't evaluate the cotangent function, because we can't divide by zero.

So, the domain of the cotangent function is composed of all the points where the sine isn't zero. And we have

And since we want to avoid these points, the domain of the cotangent will be

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