MCQ
$\cos \left[ {2{{\cos }^{ - 1}}\frac{1}{5} + {{\sin }^{ - 1}}\frac{1}{5}} \right] = $
  • A
    $\frac{{2\sqrt 6 }}{5}$
  • $ - \frac{{2\sqrt 6 }}{5}$
  • C
    $\frac{1}{5}$
  • D
    $ - \frac{1}{5}$

Answer

Correct option: B.
$ - \frac{{2\sqrt 6 }}{5}$
b
(b) $\cos \,\left( {{{\cos }^{ - 1}}\frac{1}{5} + {{\sin }^{ - 1}}\frac{1}{5} + {{\cos }^{ - 1}}\frac{1}{5}} \right) = \cos \,\left( {\frac{\pi }{2} + {{\cos }^{ - 1}}\frac{1}{5}} \right)$

$ = - \sin \,\left( {{{\cos }^{ - 1}}\frac{1}{5}} \right) = - \sin \,\left( {{{\sin }^{ - 1}}\sqrt {\frac{{24}}{{25}}} } \right) = - \frac{{2\sqrt 6 }}{5}$.

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