JEE Main & Advanced Mathematics Vector Algebra Question Bank Vector or Cross product of two vectors and its application

  • question_answer
    The sine of the angle between the two vectors \[3\mathbf{i}+2\mathbf{j}-\mathbf{k}\] and \[12\mathbf{i}+5\mathbf{j}-5\mathbf{k}\] will be            [Roorkee 1978]

    A)                 \[\frac{\sqrt{115}}{\sqrt{14}\sqrt{194}}\]       

    B)                 \[\frac{51}{\sqrt{14}\sqrt{144}}\]

    C)                 \[\frac{\sqrt{64}}{\sqrt{14}\sqrt{194}}\]         

    D)                 None of these

    Correct Answer: A

    Solution :

               \[\mathbf{a}\times \mathbf{b}=\left| \,\begin{matrix}    \mathbf{i} & \mathbf{j} & \mathbf{k}  \\    3 & 2 & -1  \\    12 & 5 & -5  \\ \end{matrix}\, \right|=-5\mathbf{i}+3\mathbf{j}-9\mathbf{k}\]                                 \[\Rightarrow \sin \theta =\frac{\sqrt{25+9+81}}{\sqrt{14}.\sqrt{194}}=\frac{\sqrt{115}}{\sqrt{14}.\sqrt{194}}\].


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