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Find the shortest distance between the lines whose vector equations are r→=(i^+2j^+3k^)+λ(i^-3j^+2k^) and r→=4i^+5j^+6k^+μ(2i^+3j^+k^).

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Question

Find the shortest distance between the lines whose vector equations are `vecr = (hati + 2hatj + 3hatk) + lambda(hati - 3hatj + 2hatk)` and `vecr = 4hati + 5hatj + 6hatk + mu(2hati + 3hatj + hatk)`.

Sum
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Solution

Compare the given equations

`vecr = vec(a_1) + λvec(b_1)` and `vecr = vec(a_2) + µvec(b_2)` respectively, we have `vec(a_1) = hati + 2hatj + 3hatk, vec(b_1) = hati - 3hatj + 2hatk`

`vec(a_2) = 4hati + 5hatj + 6hatk` and `vec(b_2) = 2hati + 3hatj + hatk`

Now, `vec(a_2) - vec(a_1) = 3hati + 3hatj + 3hatk`

And `vec(b_1) xx vec(b_2) = |(hati, hatj, hatk), (1, -3, 2), (2, 3, 1)|`

= `-9hati + 3hatj + 9hatk`

`|vec(b_1) xx vec(b_2)| = 3sqrt19`

∴ `(vec(a_2) - vec(a_1)). (vec(b_1) xx vec(b_2)) = 9`

d = `|((vec(a_2) - vec(a_1)). (vec(b_1) xx vec(b_2)))/|vec(b_1) xx vec(b_2)||`

`= |(-9 xx 3 + 3 xx 3 + 9 xx3)/sqrt((-9)^2 + 3^2 + 9^2)|`

`= |9/ (sqrt(3^2) sqrt(3^2 + 1 + 3^2))|`

= `9/(3sqrt19)`

= `3/sqrt19`

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Chapter 11: Three Dimensional Geometry - Exercise 11.2 [Page 478]

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NCERT Mathematics Part 1 and 2 [English] Class 12
Chapter 11 Three Dimensional Geometry
Exercise 11.2 | Q 16 | Page 478

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