The inverse of the point (1,2) which is found
by interchanging the two coordinates (2,1).  It
always amounts to a reflection in the 45-degree
line through the origin, the line whose equation is 
y = x.  The red curve and the green curves below
are inverses of each other.  The red curve is the
graph of f and green curve is the graph of f-1.

The dotted blue line is the line y = x, which is 
often called "the identity line".  The graph of f, 
the red curve, contains the point (1,2) and the 
graph of f-1, the green curve, contains 
the inverse point (2,1), where the coordinates are
interchanged.



Edwin