1.

Find the distance between the lines l1 and l2 with the following vector equations.\(\vec{r}=2\hat{i}+2\hat{j}-2\hat{k}+λ(3\hat{i}+2\hat{j}+5\hat{k})\)\(\vec{r}=4 \hat{i}-\hat{j}+5\hat{k}+μ(3\hat{i}-2\hat{j}+4\hat{k})\)(a) \(\frac{57}{\sqrt{47}}\)(b) \(\frac{57}{\sqrt{77}}\)(c) \(\frac{7}{\sqrt{477}}\)(d) \(\frac{57}{\sqrt{477}}\)I got this question in semester exam.The query is from Three Dimensional Geometry in section Three Dimensional Geometry of Mathematics – Class 12

Answer»

Right option is (d) \(\frac{57}{\sqrt{477}}\)

To EXPLAIN I would SAY: We know that, the shortest distance between two skew lines is given by

d=\(\LEFT |\frac{(\vec{b_1}×\vec{b_2}).(a_2-a_1)}{|\vec{b_1}×\vec{b_2}|}\right |\)

The VECTOR EQUATIONS of the two lines is

\(\vec{r}=2\hat{i}+2\hat{j}-2\hat{k}+λ(3\hat{i}+2\hat{j}+5\hat{k})\)

\(\vec{r}=4 \hat{i}-\hat{j}+5\hat{k}+μ(3\hat{i}-2\hat{j}+4\hat{k})\)

∴d=\(\left|\frac{((3\hat{i}+2\hat{j}+5\hat{k})×(3\hat{i}-2\hat{j}+4\hat{k}).(4\hat{i}-\hat{j}+5\hat{k})-(2\hat{i}+2\hat{j}-2\hat{k}))}{|(3\hat{i}+2\hat{j}+5\hat{k})×(3\hat{i}-2\hat{j}+4\hat{k})|}\right |\)

\((3\hat{i}+2\hat{j}+5\hat{k})×(3\hat{i}-2\hat{j}+4\hat{k})=\begin{vmatrix}\hat{i}&\hat{j}&\hat{k}\\3&2&5\\3&-2&4\end{vmatrix}\)

=\(\hat{i}(8+10)-\hat{j}(12-15)+\hat{k}(-6-6)\)

=\(18\hat{i}+3\hat{j}-12\hat{k}\)

d=\(\left|\frac{(18\hat{i}+3\hat{j}-12\hat{k}).(2\hat{i}-3\hat{j}+7\hat{k})}{\sqrt{18^2+3^2+(-12)^2}}\right |\)

d=\(\left|\frac{36-9-84}{\sqrt{477}}\right |\)=\(\frac{57}{\sqrt{477}}\).



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