MCQ
If $\mathop {\lim }\limits_{n \to \infty } \frac{{{1^a} + {2^a} + ....... + {n^a}}}{{{{\left( {n + 1} \right)}^{a - 1}}\left[ {\left( {na + 2} \right) + ......\left( {na + n} \right)} \right]}} = \frac{1}{{60}}$ for some positive real number $a$, then $a$ is equal to 
  • $7$
  • B
    $8$
  • C
    $\frac{15}{2}$
  • D
    $\frac{17}{2}$

Answer

Correct option: A.
$7$
a
$\,\mathop {\lim }\limits_{n \to \infty } \frac{{\frac{1}{{\left( {a + 1} \right)}}{n^{a + 1}} + {a_1}{n^a} + {a_2}{n^{a - 1}} + .......}}{{{{\left( {n + 1} \right)}^{a - 1}}.{n^2}\left( {a + \frac{{1 + \frac{1}{n}}}{2}} \right)}} = \frac{1}{{60}}$

$\,\mathop {\lim }\limits_{n \to \infty } \frac{{{{\left( {\frac{1}{n}} \right)}^a} + {{\left( {\frac{2}{n}} \right)}^a} + ..... + {{\left( {\frac{n}{n}} \right)}^a}}}{{{{\left( {n + 1} \right)}^{a - 1}}\left[ {{n^a} + \frac{{n\left( {n + 1} \right)}}{2}} \right]}}.$

$ = \,\frac{{\mathop {\lim }\limits_{n \to \infty } \frac{1}{n}\sum\limits_{r = 1}^n {{{\left( {\frac{r}{n}} \right)}^a}} }}{{{{\left( {1 + \frac{1}{n}} \right)}^{a - 1}}\left[ {a + \frac{1}{2}\left( {1 + \frac{1}{n}} \right)} \right]}}\, = \frac{1}{{60}}$

$ = \frac{{\int\limits_0^1 {{x^n}dx} }}{{\left( {a + \frac{1}{2}} \right)}} = \frac{1}{{60}} = \frac{{\frac{1}{{a + 1}}}}{{a + \frac{1}{2}}} = \frac{1}{{60}}$

$ \Rightarrow \frac{{\frac{1}{{a + 1}}}}{{\left( {a + \frac{1}{2}} \right)}} = \frac{1}{{60}} \Rightarrow \left( {a + 1} \right)\left( {2a + 1} \right) = 120$

$ \Rightarrow 2{a^2} + 3a - 119 = 0$

$ \Rightarrow 2{a^2} + 17a - 14a - 119 = 0$

$ \Rightarrow \left( {a - 7} \right)\left( {2a + 17} \right) = 0$

                                               $a = 7, - \frac{{17}}{2}$

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