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单词 ProofOfLimitRuleOfProduct
释义

proof of limit rule of product


Let f and g be real (http://planetmath.org/RealFunction) or complex functions having the limits

limxx0f(x)=Fandlimxx0g(x)=G.

Then also the limit limxx0f(x)g(x) exists and equals FG.

Proof.  Let ε be any positive number.  The assumptionsPlanetmathPlanetmath imply the existence of the positive numbers δ1,δ2,δ3 such that

|f(x)-F|<ε2(1+|G|)when  0<|x-x0|<δ1(1)
|g(x)-G|<ε2(1+|F|)when  0<|x-x0|<δ2,(2)
|g(x)-G|<1when  0<|x-x0|<δ3.(3)

According to the condition (3) we see that

|g(x)|=|g(x)-G+G||g(x)-G|+|G|<1+|G|when  0<|x-x0|<δ3.

Supposing then that  0<|x-x0|<min{δ1,δ2,δ3}  and using (1) and (2) we obtain

|f(x)g(x)-FG|=|f(x)g(x)-Fg(x)+Fg(x)-FG|
|f(x)g(x)-Fg(x)|+|Fg(x)-FG|
=|g(x)||f(x)-F|+|F||g(x)-G|
<(1+|G|)ε2(1+|G|)+(1+|F|)ε2(1+|F|)
=ε

This settles the proof.

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更新时间:2025/5/4 1:45:44