The First Fundamental Form
In a biography published several years after Riemann's death, fellow German mathematician Hans Freundenthal (1905-1990) described this Riemann surface as one that
posesses shortest lines, now called geodesics, which resemble ordinary straight lines. In fact, at first approximation in a geodesic coordinate system such a metric is flat in the same way that a curved surface up to higher-order terms looks like its tangent plane. Beings living on the surface may discover the curvature of their world and compute it at any point as a consequence of observed deviations from Pythagoras' theorem.
In order to calculate the length of a straight line on a plane, thus, we may use the equation $$A^2+B^2=C^2$$.
In order to calculate the length of a line on a curve, however, we use an equation referred to as the first fundamental form:
where the coefficients are given by:
For our purposes, it isn't important that we memorize this equation. Rather, simply realize that it exists, and understand what it is used for.