To The Absolute Limit

Calculus Level 1

lim x 1 x 1 x 1 = ? \large \lim_{x \to 1} \dfrac{|x-1|}{x-1} = \, ?

1 -1 0 Does not exist

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2 solutions

To solve this problem, we consider the two cases for absolute value. If x 1 < 0 x - 1 < 0 , we can replace x 1 |x - 1| with ( x 1 ) -(x-1) . We can see that as x approaches 1 from the left, x - 1 approaches 0 from the left. Therefore,

lim x 1 x 1 x 1 = lim x 1 ( x 1 ) x 1 = 1 \lim_{x \to 1^-} \frac{|x-1|}{x-1} = \lim_{x \to 1^-} \frac{-(x-1)}{x-1} = -1

Similarly, for the case where x 1 > 0 x - 1 > 0 , we have x 1 = x 1 |x - 1| = x - 1 .

lim x 1 + x 1 x 1 = lim x 1 + x 1 x 1 = 1 \lim_{x \to 1^+} \frac{|x-1|}{x-1} = \lim_{x \to 1^+} \frac{x-1}{x-1} = 1

Since the left hand limit does not equal the right hand limit, the limit does not exist.

Jack Gee
Feb 8, 2016

L'Hospital's principle can be used, I think?

d y d x x \frac {dy}{dx} |x| does not exist for x = 0 x=0

Let u = x 1 u = x-1

Finding the new limit:

x 1 u 0 x \rightarrow 1 \Rightarrow u \rightarrow 0

From knowing that the derivative of the modulus of some variable doesn't exist when the variable is equal to zero; the limit equally doesn't exist. L'Hospital's principle would suggest that the limit also doesn't exist?

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