JEE MAIN - Mathematics (2023 - 15th April Morning Shift - No. 13)
Let $\mathrm{S}$ be the set of all values of $\lambda$, for which the shortest distance between
the lines $\frac{x-\lambda}{0}=\frac{y-3}{4}=\frac{z+6}{1}$ and $\frac{x+\lambda}{3}=\frac{y}{-4}=\frac{z-6}{0}$ is 13. Then $8\left|\sum\limits_{\lambda \in S} \lambda\right|$ is equal to :
the lines $\frac{x-\lambda}{0}=\frac{y-3}{4}=\frac{z+6}{1}$ and $\frac{x+\lambda}{3}=\frac{y}{-4}=\frac{z-6}{0}$ is 13. Then $8\left|\sum\limits_{\lambda \in S} \lambda\right|$ is equal to :
306
304
308
302
Explanation
Given the two lines :
$$ \frac{x-\lambda}{0}=\frac{y-3}{4}=\frac{z+6}{1} \\\\ \frac{x+\lambda}{3}=\frac{y}{-4}=\frac{z-6}{0} $$
Let's find the direction vectors of these lines: $\vec{d_1} = \langle 0, 4, 1 \rangle$ and $\vec{d_2} = \langle 3, -4, 0 \rangle$.
Now, let's find the cross product of the direction vectors, which will give a vector that is perpendicular to both lines :
$\vec{n} = \vec{d_1} \times \vec{d_2} = \langle 4, 3, -12 \rangle$
Let's find the vector connecting a point on line 1 to a point on line 2 :
$\vec{c} = \langle 2\lambda, 3, -12 \rangle$
The shortest distance between the two lines is the projection of $\vec{c}$ onto $\vec{n}$ :
$d = \left|\frac{\vec{c} \cdot \vec{n}}{|\vec{n}|}\right| = \left|\frac{(2\lambda)(4) + (3)(3) - (12)(-12)}{\sqrt{16 + 9 + 144}}\right|$
We are given that the shortest distance is 13 :
$13 = \left|\frac{8\lambda + 153}{13}\right|$
$|8\lambda + 153| = 169$
We have two cases :
1. $8\lambda + 153 = 169$
$\lambda = \frac{16}{8}$
2. $8\lambda + 153 = -169$
$\lambda = \frac{-322}{8}$
Now, let's calculate $8\left|\sum\limits_{\lambda \in S} \lambda\right|$ :
$8\left|\frac{16}{8} + \frac{-322}{8}\right| = 8\left|\frac{-306}{8}\right| = 306$
Thus, the correct answer is Option A : 306.
$$ \frac{x-\lambda}{0}=\frac{y-3}{4}=\frac{z+6}{1} \\\\ \frac{x+\lambda}{3}=\frac{y}{-4}=\frac{z-6}{0} $$
Let's find the direction vectors of these lines: $\vec{d_1} = \langle 0, 4, 1 \rangle$ and $\vec{d_2} = \langle 3, -4, 0 \rangle$.
Now, let's find the cross product of the direction vectors, which will give a vector that is perpendicular to both lines :
$\vec{n} = \vec{d_1} \times \vec{d_2} = \langle 4, 3, -12 \rangle$
Let's find the vector connecting a point on line 1 to a point on line 2 :
$\vec{c} = \langle 2\lambda, 3, -12 \rangle$
The shortest distance between the two lines is the projection of $\vec{c}$ onto $\vec{n}$ :
$d = \left|\frac{\vec{c} \cdot \vec{n}}{|\vec{n}|}\right| = \left|\frac{(2\lambda)(4) + (3)(3) - (12)(-12)}{\sqrt{16 + 9 + 144}}\right|$
We are given that the shortest distance is 13 :
$13 = \left|\frac{8\lambda + 153}{13}\right|$
$|8\lambda + 153| = 169$
We have two cases :
1. $8\lambda + 153 = 169$
$\lambda = \frac{16}{8}$
2. $8\lambda + 153 = -169$
$\lambda = \frac{-322}{8}$
Now, let's calculate $8\left|\sum\limits_{\lambda \in S} \lambda\right|$ :
$8\left|\frac{16}{8} + \frac{-322}{8}\right| = 8\left|\frac{-306}{8}\right| = 306$
Thus, the correct answer is Option A : 306.
Comments (0)
