39 The plane figures with which we are concerned come mainly under the description of graphs of continuous variation.
In particular, the equality or inequality of values of two functions is more readily grasped by comparison of the lengths of the ordinates of the graphs than by inspection of the relative positions of their bounding lines.
The second class of cases comprises equations involving two unknowns; here we have to deal with two graphs, and the solution of the equation is the determination of their common ordinates.
[[Mensuration Of Graphs 38.]] (A) Preliminary.
(B) Mensuration of Graphs of Algebraical Functions.