Chord length |AB| =|t 1-t2|
|PB|x|PA|=|t 1 x t2|？
|PB|+|PA|=|t 1|+|t2|
Polar coordinate system is especially useful when the relationship between two points can be easily expressed by included angle and distance. In the plane rectangular coordinate system, such a relationship can only be expressed by trigonometric functions.
Cauchy mean value theorem
If the functions f(x) and F(x) satisfy:
(1) is continuous on the closed interval [a, b].
⑵ In the open interval (a, b), it is derivable.
(3) For any x∈(a, b), F'(x)≠0.
Then at least one point ζ in (a, b) constitutes the equation.
[f (b)-f (a)]/[f (b)-f (a)] = f' (zeta)/f' (zeta) holds.