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NCERT Class XI Mathematics - Limits and Derivatives - Solutions

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Question : 30 of 72
Marks: +1, -0
If f (x) = {x+1,x<00,x=0x1,x>0\begin{cases} |x| + 1, & x < 0 \\ 0, & x = 0 \\ |x| - 1, & x > 0 \end{cases}. For what value(s) of a does limxa\lim\limits_{x \to a} f (x) exists ?
Solution:  
We have f (x) = {x+1,x<00,x=0x1,x>0\begin{cases} |x| + 1, & x < 0 \\ 0, & x = 0 \\ |x| - 1, & x > 0 \end{cases}
Case (i) : When a < 0, we have
limxa\lim\limits_{x \to a} f (x) = limxa\lim\limits_{x \to a^{-}} |x| + 1 = limxa\lim\limits_{x \to a^{-}} (- x + 1) = - a + 1
Case (ii) : When a > 0, we have limxa\lim\limits_{x \to a} f (x) = limxa+\lim\limits_{x \to a^{+}} (x - 1) = a - 1
Case (iii) : When a = 0, we have
= limxa+\lim\limits_{x \to a^{+}} f (x) = limx0+\lim\limits_{x \to 0^{+}} f (x) = limx0+\lim\limits_{x \to 0^{+}} (|x| - 1) = 0 - 1 = - 1
limxa\lim\limits_{x \to a^{-}} = limx0\lim\limits_{x \to 0^{-}} f (x) = limx0\lim\limits_{x \to 0^{-}} (|x| + 1) = - 0 + 1 = 1
limxa+\lim\limits_{x \to a^{+}} f (x) ≠ limxa\lim\limits_{x \to a^{-}} f (0) [Since f (0) = 0]
limxa\lim\limits_{x \to a} f (x) does not exist when a = 0.
limxa\lim\limits_{x \to a} f (x) exists for all a ≠ 0.
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