Prove: (f(x+g(x))−f(g(x)))/(x) -> f′(0) as x approaches 0

Rene Nicholson

Rene Nicholson

Answered question

2022-10-30

Prove: f ( x + g ( x ) ) f ( g ( x ) ) x f ( 0 ) as x approaches 0

Answer & Explanation

Jimena Torres

Jimena Torres

Beginner2022-10-31Added 20 answers

Given ϵ > 0 there is a δ > 0 such that
| f ( x ) f ( 0 ) x f ( 0 ) | < ϵ | x |
for | x | < δ
Now choose | x | < δ / 2. Then | g ( x ) | < δ and | x + g ( x ) | < δ, and therefore
| f ( x + g ( x ) ) f ( g ( x ) ) x f ( 0 ) | = | f ( x + g ( x ) ) f ( 0 ) ( x + g ( x ) ) f ( 0 ) x f ( g ( x ) ) f ( 0 ) g ( x ) f ( 0 ) x | | f ( x + g ( x ) ) f ( 0 ) ( x + g ( x ) ) f ( 0 ) | | x | + | f ( g ( x ) ) f ( 0 ) g ( x ) f ( 0 ) | | x | ϵ | x + g ( x ) | | x | + ϵ | g ( x ) | | x | 3 ϵ .

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