MCQ
If $\mathop {\lim }\limits_{x \to a} \frac{{{a^x} - {x^a}}}{{{x^x} - {a^a}}} = - 1$, then
  • $a = 1$
  • B
    $a = 0$
  • C
    $a = e$
  • D
    None of these

Answer

Correct option: A.
$a = 1$
a
(a) Using $ L-$ Hospital's rule, we get

$ - 1 = \mathop {\lim }\limits_{x \to a} \,\,\frac{{{a^x} - {x^a}}}{{{x^x} - {a^a}}} = \mathop {\lim }\limits_{x \to a} \,\frac{{{a^x}{{\log }_e}a - a{x^{a - 1}}}}{{{x^x} + {a^a}{{\log }_e}a}}$

$ \Rightarrow \,\, - 1 = \frac{{{a^a}{{\log }_e}a - a\,.\,{a^{a - 1}}}}{{{a^a} + {a^a}{{\log }_e}a}} = \frac{{{{\log }_e}a - 1}}{{{{\log }_e}a + 1}}$.....$(i)$

Now $(i)$ is satisfied only when $a = 1.$

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