Show that two primitives/ antiderivatives are related via a constant Let I &#x2282;<!-- ⊂ -->

Leah Pope

Leah Pope

Answered question

2022-06-24

Show that two primitives/ antiderivatives are related via a constant
Let I R be an interval. A differentiable function F : I R is called a primitive for the function f : I R if F ( x ) = f ( x ) for all x I.
Show: If F 1 and F 2 are two primitives for f on I then there is a constant C R such that F 2 F 1 + C, i.e. F 2 ( x ) = F 1 ( x ) + C for all x I.
What we have covered so far is the formal definition of the derivative in terms of the limit:
lim h 0 F 1 ( x + h ) F 1 ( x ) h = f ( x )
lim h 0 F 2 ( x + h ) F 2 ( x ) h = f ( x )
I do not see how I can prove firsthand that these two functions differ by a constant. I know it to be true from my pre-calculus and calculus experience, but how would one make the argument from an analysis point of view, can someone give me a hint as to where this constant 'appears'.
We normally just define F 2 = F 1 + C and then it would follow immediately that these two functions have the same derivative.

Answer & Explanation

candelo6a

candelo6a

Beginner2022-06-25Added 24 answers

Step 1
Let G ( x ) = F 1 F 2 ..
We observe that G ( x ) = 0. We claim this means that G(x) is constant.
Suppose on the contrary the function is non-constant this means there must exist a , b I and where a < b such that G ( a ) G ( b ).
Step 2
We consider [a,b]. Now We apply the mean value theorem to this interval and the function G(x). We get that there must exist a c such that: Which is a contradiction.
G ( c ) = G ( b ) G ( a ) b a G ( c ) = 0 ,
G ( b ) G ( a ) b a = 0 G ( a ) = G ( b ) .
We conclude that G(x) is constant. Thus, F 2 and F 1 differ by a constant .
Alannah Short

Alannah Short

Beginner2022-06-26Added 5 answers

Explanation:
If F 1 , F 2 are primitives of f, by linearity of differentiation,
( F 1 ( x ) F 2 ( x ) ) = F 1 ( x ) F 2 ( x ) = f ( x ) f ( x ) = 0.
Remains to prove that the antiderivative of 0 is a constant function.

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