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G. Constant Difference Thm
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the Real Analysis appendices of

Why Slopes
and
More Math

Volume 3

Printed in Canada
ISBN 0-9697564-3-7

These  Real Analysis appendices continue the decimal viewpoint of limits, continuity and convergence in chapter 14. and this further lesson

A. What's Next
B. Pigeon Hole Principle
B. Bolzano-Weierstrass
C1. Triangle Inequality
C2. Triangle Inequality
C. More T.Inequality
D. Sets & Sequences
D. Monotone Sequences
E. Limits,  Properties
E Limits & Error Control
F. Continuous Functions
F. Closed Range Thm
F. Intermediate Val. Thm
F. Compactness Thm
F. Equicontinuity Thm
F Extreme Value Thm
G. Rolle's Theorem etc
G. Mean Val. Thm.
G. Constant Difference Thm
G. Lipschitz Continuity I
PS: One Sided Range Theorems
G. Velocity Revisited
G. Sufficient Conditions
H. Riemann Sums Conv
H. Lipschitz Continuity II

Proofs of  one-sided theorems could be of interest in the study of 2D topology.

Constant Difference Theorem Again

We are now in a position to restate and prove the constant difference theorem:

Theorem G.3 [Constant Difference Theorem] If f1(x) and f2(x) have the same non-infinite slope (that is, derivative) m = f¢1(x) = f¢2(x) at every x in an interval (a,b) then the difference
f2(x)-f1(x) = d
is constant for a < x < b. That is, there is a constant d such that 
f2(x) = f1(x) +d
for every x in the interval (a,b). This number d does not depend on x.

Proof of Constant Difference Theorem.

Put f(x) = f1(x)-f2(x). Then f¢(x) = f¢1(x)-f¢2(x) for every x interior to the interval [a,b and f is continuous on the interval. Now f¢(x) = 0 interior to the interval since f¢1(x) = f¢2(x). Now suppose a < x1 £ b. (Suggestion take x1 = b on first reading.) The mean value theorem applied to each subinterval [a,x1] implies there exist a c in the interval [a,x1] such that 
f(x1)-f(a)
x1-a
= f¢(c)
But f¢(c) = 0. Therefore f(x1) = f(a). The latter holds for a < x1 £ b.

Recall the role of the constant difference theorem in the chapter Slopes and Areas. This theorem implied the area under a (continuous) curve y = f(x) from x = a to x = b is given by G(b)-G(a) whenever G¢(x) = f(x) for a < x < b and G(x) is continuous on [a,b].

 

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