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Find the shortest distance between the lines: r→ =(i^+2j^+k^)+λ(i^-j^+k^) and r→=2i^-j^-k^+μ(2i^+j^+2k^)

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Question

Find the shortest distance between the lines: 

`vecr = (hati+2hatj+hatk) + lambda(hati-hatj+hatk)` and `vecr = 2hati - hatj - hatk + mu(2hati + hatj + 2hatk)`

Sum
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Solution

Given the equation of lines `vecr = (hati + 2hatj + hatk) + λ(hati - hatj + hatk)`       .....(1)

and `vecr = 2hati - hatj - hatk + µ(2hati + hatj + 2hatk)`     .....(2)

Comparing equation (1) with `vecr = vec(a_1) + λvec(b_1)` and equation (2) with `vecr = vec(a_2) + µvec(b_2)`,

`vec(a_1) = hati + 2hatj + hatk`  and  `vec(a_2) = 2hati - 2hatj - hatk` 

`vec(b_1) = hati - hatj + hatk`  and  `vec(b_2) = 2hati + hatj + 2hatk` 

Now, `vec(a_2) - vec(a_1) = hati - 3hatj - 2hatk`

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

= `(-2 -1) hati - (2 - 2) hatj + (1 + 2) hatk`

= `-3hati - 0hatj + 3hatk`

Required shortest distance

`= |((vecb_1 xx vecb_2)* (veca_2 - veca_1))/|vecb_1 xx vecb_2||`

`= |((-3hati + 3hatk)* (hati - 3hatj - 2hatk))/|-3hati + 3hatk||`

`= |((-3) xx 1 + 0 xx (-3) + 3 xx (-2))/ sqrt((-3)^2 + 3^2)|`

`= 9/sqrt18`

`= 9/(3sqrt2)`

`= 3/sqrt2` units

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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 14 | Page 478

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