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Question: Answered & Verified by Expert
If the shortest distance between the lines $\mathbf{r}=(3 \hat{\mathbf{i}}+4 \hat{\mathbf{j}}-2 \hat{\mathbf{k}})+t(-\hat{\mathbf{i}}+2 \hat{\mathbf{j}}+\hat{\mathbf{k}})$ and $\mathbf{r}=(\hat{\mathbf{i}}-7 \hat{\mathbf{j}}-2 \hat{\mathbf{k}})+s(\hat{\mathbf{i}}+3 \hat{\mathbf{j}}+2 \hat{\mathbf{k}})$ is equivalent to projection of $\mathbf{P}=-2 \hat{\mathbf{i}}+11 \hat{\mathbf{j}}$ on $\mathbf{Q}$ then a possible vector $\mathbf{Q}$ is
MathematicsThree Dimensional GeometryTS EAMCETTS EAMCET 2020 (09 Sep Shift 2)
Options:
  • A $\hat{i}+5 \hat{j}-3 \hat{k}$
  • B $5 \hat{i}-\hat{j}+3 \hat{k}$
  • C $\hat{\mathrm{i}}+3 \hat{\mathrm{j}}+5 \hat{\mathrm{k}}$
  • D $3 \hat{i}+5 \hat{j}-\hat{k}$
Solution:
1110 Upvotes Verified Answer
The correct answer is: $\hat{\mathrm{i}}+3 \hat{\mathrm{j}}+5 \hat{\mathrm{k}}$
The shortest distance between given lines $\mathbf{r}=(3 \hat{\mathbf{i}}+4 \hat{\mathbf{j}}-2 \hat{\mathbf{k}})+t(-\hat{\mathbf{i}}+2 \hat{\mathbf{j}}+\hat{\mathbf{k}})$ and $\mathbf{r}=(\hat{\mathbf{i}}-7 \hat{\mathbf{j}}-2 \hat{\mathbf{k}})+s(\hat{\mathbf{i}}+3 \hat{\mathbf{j}}+2 \hat{\mathbf{k}})$ is
$\begin{aligned} d & =\frac{\left|\begin{array}{ccc}2 & 11 & 0 \\ -1 & 2 & 1 \\ 1 & 3 & 2\end{array}\right|}{\sqrt{(4-3)^2+(-2-1)^2+(-3-2)^2}} \\ & =\frac{|2(4-3)-11(-2-1)|}{\sqrt{1+9+25}}=\frac{35}{\sqrt{35}}=\sqrt{35}\end{aligned}$
Now the projection of $\mathbf{p}=-2 \hat{\mathbf{i}}+11 \hat{\mathbf{j}}$ on vector $\mathbf{Q}=-\hat{\mathbf{i}}+3 \hat{\mathbf{j}}+5 \hat{\mathbf{k}}$ is also $\sqrt{35}$
In option (c) inside of $\hat{\mathbf{i}}+3 \hat{\mathbf{j}}+5 \hat{\mathbf{k}}$ put $-\hat{\mathbf{i}}+3 \hat{\mathbf{j}}+5 \hat{\mathbf{k}}$ then option (c) is ok

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