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Racetrack principle : ウィキペディア英語版
Racetrack principle

In calculus,
the racetrack principle describes the movement and growth of two functions in terms of their derivatives.
This principle is derived from the fact that if a horse named Frank Fleetfeet always runs faster than a horse named Greg Gooseleg, then if Frank and Greg start a race from the same place and the same time, then Frank will win. More briefly, the horse that starts fast and stays fast wins.
In symbols:
:if f'(x)>g'(x) for all x>0, and if f(0)=g(0), then f(x)>g(x) for all x>0.
or, substituting ≥ for > produces the theorem
:if f'(x) \ge g'(x) for all x>0, and if f(0)=g(0), then f(x) \ge g(x) for all x>0.
which can be proved in a similar way
==Proof==
This principle can be proven by considering the function h(x) = f(x) - g(x). If we were to take the derivative we would notice that for x>0
: h'= f'-g'>0.
Also notice that h(0) = 0. Combining these observations, we can use the mean value theorem on the interval (x ) and get
: h'(x_0)= \frac= \frac>0.
Since x > 0 for the mean value theorem to work then we may conclude that f(x) - g(x) > 0. This implies f(x) > g(x).

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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