The first kind of function is a constant function.

If *f*(*x*) = *C* for some constant *C*, then *f'*(*x*) = 0.

Constant functions have a slope of zero. On a graph, a constant function is a straight vertical line. It doesn't matter if we pick *f*(*x*) = 3 or *f*(*x*) = -10, or *f*(*x*) = π, or any other constant, the result would be the same. Zero. We like.

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