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Write a proof for each limit using the εδ\varepsilon-\delta definition of a limit. limx3(4x)=12\lim _{x \rightarrow 3}(4 x)=12

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We would like to proof for the following limit by using the (δ, ϵ) definition of limit.\text{\color{#4257b2}We would like to proof for the following limit by using the\ $(\delta,\ \epsilon)$ definition of limit.}

limx3 (4x)=12\color{Brown}\lim\limits_{x\to3}\ (4x)=12

\lozenge\ \ Solution:\text{\underline{\bf{Solution:}}}

Assume that \ \ δ>0,      ϵ>0\delta>0,\ \ \ \ \ \ \epsilon>0

There is a general rule in the definition of limit:

0<xa<δ      f(x)L<ϵ0<|x-a|<\delta\ \ \ \Rightarrow\ \ \ |f(x)-L|<\epsilon

Apply this rule for the given data as follows:

x3<δ      4x12<ϵ|x-3|<\delta\ \ \ \Rightarrow\ \ \ |4x-12|<\epsilon

Use GCF property as follows:

x3<δ      4(x3)<ϵ|x-3|<\delta\ \ \ \Rightarrow\ \ \ |4(x-3)|<\epsilon

x3<δ      4x3<ϵ|x-3|<\delta\ \ \ \Rightarrow\ \ \ 4|x-3|<\epsilon

Divide the right side by (4)(4) as follows:

x3<δ      x3<ϵ4|x-3|<\delta\ \ \ \Rightarrow\ \ \ |x-3|<\dfrac{\epsilon}{4}

Since the absolute value in the both side are equal, so the result values are equals too:

  x3=x3                   δ= ϵ4 \because\ \ |x-3|=|x-3|\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \therefore\ \ \ \delta=\boxed{\ \dfrac{\epsilon}{4}\ }

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