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Railways Sample Paper RRBs Assistant Loco Pilot and Technician CBT STAGE-I Sample Paper-1

  • question_answer
    If \[\sin B=\,\frac{1}{2}\,,\]then \[3\cos B-4{{\cos }^{3}}B\]is equal to

    A)  1                    

    B)  \[\frac{3}{4}\]

    C)  0                    

    D)  21

    Correct Answer: C

    Solution :

    \[\sin \,B\,\,=\,\,\frac{1}{2}\] Then, \[\cos B=\sqrt{1-{{\sin }^{2}}B}=\sqrt{1-{{\left( \frac{1}{2} \right)}^{2}}}=\frac{\sqrt{3}}{2}\] \[\therefore \] . \[\,=\frac{3\sqrt{3}}{2}-\frac{4\times 3\sqrt{3}}{8}\] \[=\frac{3\sqrt{3}}{2}-\frac{3\sqrt{3}}{12}=0\]


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