Copyright © University of Cambridge. All rights reserved.

Draw some sketches of familiar curves and note that the condition
in the question can be interpreted geometrically as the function
and the gradient having opposite signs at each point along the
curve.

So, the question can be rephrased as: the sign of a curve and the sign of its gradient be opposite everywhere?

So, the question can be rephrased as: the sign of a curve and the sign of its gradient be opposite everywhere?