JEE Main & Advanced Mathematics Three Dimensional Geometry Question Bank System of Co-ordinates, Direction cosines and direction ratios, Projection

  • question_answer
    If a,b,g  be the direction angles of a vector and \[\cos \alpha =\frac{14}{15}\], \[\cos \beta =\frac{1}{3}\] then \[\cos \gamma \]=

    A)            \[\pm \frac{2}{15}\]

    B)            \[\frac{1}{5}\]

    C)            \[\pm \frac{1}{15}\]

    D)            None of these

    Correct Answer: A

    Solution :

               \[{{\cos }^{2}}\alpha +{{\cos }^{2}}\beta +{{\cos }^{2}}\gamma =1\]                    \[\Rightarrow \,\,\cos \,\gamma =\sqrt{1-{{\left( \frac{14}{15} \right)}^{2}}-{{\left( \frac{1}{3} \right)}^{2}}}\]\[=\sqrt{\frac{8}{9}-\left( \frac{196}{225} \right)}=\pm \frac{2}{15}\].


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