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[SOLVED] Simple question about Limit Properties

Cbarker1

Active member
Jan 8, 2013
242
I read a textbook about limits. I saw several properties about the limits.

\(\displaystyle lim f(bx)= b*lim f(x)\) as x approach c


Thank you

Cbarker1
 

Guest

Active member
Jan 4, 2014
199
Pick $ f(y) = \frac{1}{y}$. Then $\displaystyle \lim_{x \to c} f(bx) = \lim_{x \to c}\frac{1}{bx} = \frac{1}{bc}$. On the other hand $\displaystyle \lim_{x \to c} bf(x) = b\lim_{x \to c}\frac{1}{x} = \frac{b}{c}.$

Of course $ \frac{1}{bc} \ne \frac{b}{c}$. I'm missing something or the property is false (or they meant something else).
 

soroban

Well-known member
Feb 2, 2012
409
Hello, Cbarker1!

I read a textbook about limits.
I saw several properties about the limits.

\(\displaystyle \lim_{x\to c} f(bx)\:=\: b\cdot\lim_{x\to c}f(x)\)
This is not true.

Let [tex]f(x) \:=\:2x+7[/tex]

Then: .[tex]\lim_{x\to1}f(3x) \:=\:\lim_{x\to1}(6x+7) \:=\:13[/tex]

But: .[tex]3\cdot\lim_{x\to1}f(x) \:=\:3\cdot\lim_{x\to1}(2x+7) \:=\:3\cdot 9 \:=\:27[/tex]


Perhaps you misread the identity.
The following is true.

. . [tex]\lim_{x\to c}b\!\cdot\!\!f(x) \;=\;b\!\cdot\!\lim_{x\to c}f(x)[/tex]