VIT Engineering VIT Engineering Solved Paper-2015

  • question_answer
    The shortest distance between the lines  \[\frac{\text{x-7}}{\text{3}}\text{=}\frac{\text{y+4}}{\text{-16}}\text{=}\frac{\text{z-6}}{\text{7}}\]and \[\frac{\text{x-10}}{\text{3}}\text{=}\frac{\text{y-30}}{\text{8}}\text{=}\frac{\text{4-z}}{\text{5}}\]  is

    A)  \[\frac{234}{7}units\]                   

    B)  \[\frac{288}{21}units\]

    C) \[\frac{221}{3}units\]                    

    D) \[\frac{234}{21}units\]

    Correct Answer: B

    Solution :

     Given, lines are  \[\frac{x-7}{3}=\frac{y+4}{-16}=\frac{z-6}{7}\]and\[\frac{x-10}{3}=\frac{y-30}{8}=\frac{4-z}{5}\] The vector form of given equation are\[r=7\hat{i}-4\hat{j}+6\hat{k}+\lambda (3\hat{i}-16\hat{j}+7\hat{k})\]  and \[r=10\hat{i}+30\hat{j}+4\hat{k}+\mu (3\hat{i}+8\hat{j}-5\hat{k})\] On comparing these equation with                 \[r={{a}_{1}}+\lambda {{b}_{1}}\]and\[r={{a}_{2}}+\mu {{b}_{2}},\]we get                 \[{{a}_{1}}=7\hat{i}-4\hat{j}+6\hat{k}\]                 \[{{a}_{2}}=10\hat{i}+30\hat{j}+4\hat{k}\]                 \[{{b}_{1}}=3\hat{i}-16\hat{j}+7\hat{k}\]and\[{{b}_{2}}=3\hat{i}+8\hat{j}-5\hat{k}\]                 Now,\[({{a}_{2}}-{{a}_{1}})\]                 \[=10\hat{i}+30\hat{j}+4\hat{k})-(7\hat{i}-4\hat{j}+6\hat{k})\]                 \[=3\hat{i}+34\hat{j}-2\hat{k}\]                 \[{{b}_{1}}\times {{b}_{2}}=\left| \begin{matrix}    {\hat{i}} & {\hat{j}} & {\hat{k}}  \\    3 & -16 & 7  \\    3 & 8 & -5  \\ \end{matrix} \right|\]                 \[=(80-56)\hat{i}+(21+15)\hat{j}+(24+48)\hat{k}\] \[=24\hat{i}+36\hat{j}+72\hat{k}\]                 Then\[|{{b}_{1}}\times {{b}_{2}}|=\sqrt{{{(24)}^{2}}+{{(36)}^{2}}+{{(72)}^{2}}}\]                 \[=\sqrt{576+1296+5184}\]                 \[=\sqrt{7056}=84\] \[\therefore \]Shortest distance\[=\frac{100000}{99}\approx 1010\]            \[\Rightarrow \]             \[{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{6}}}\left( \text{g} \right)\text{=}{{\text{p}}_{\text{1}}}\]


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