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Consider the line r⃗=−2i^+3j^+k^+λ(5i^−3j^−k^)\vec{r} = -2\hat{i} + 3\hat{j} + \hat{k} + \lambda(5\hat{i} - 3\hat{j} - \hat{k}). Match List-I with List-II

List-IList-II
(A) A point on the given line(I) (535,−335,−135)\left(\frac{5}{\sqrt{35}}, \frac{-3}{\sqrt{35}}, \frac{-1}{\sqrt{35}}\right)
(B) Direction ratios of the given line(II) (2, 3, 1)
(C) Direction cosines of the given line(III) (5, -3, -1)
(D) Direction ratios of a line perpendicular to given line(IV) (-2, 3, 1)

Choose the correct answer from the options given below:

Solution

✅ Correct Option: 2

The line is given as: r⃗=−2i^+3j^+k^+λ(5i^−3j^−k^)\vec{r} = -2\hat{i} + 3\hat{j} + \hat{k} + \lambda(5\hat{i} - 3\hat{j} - \hat{k})

This is in the vector form r⃗=a⃗+λb⃗\vec{r} = \vec{a} + \lambda\vec{b} where:

a⃗=−2i^+3j^+k^\vec{a} = -2\hat{i} + 3\hat{j} + \hat{k} gives a point on the line

b⃗=5i^−3j^−k^\vec{b} = 5\hat{i} - 3\hat{j} - \hat{k} gives the direction of the line


(A) A point on the given line → (IV) (-2, 3, 1)

Setting λ=0\lambda = 0:

r⃗=−2i^+3j^+k^+0(5i^−3j^−k^)\vec{r} = -2\hat{i} + 3\hat{j} + \hat{k} + 0(5\hat{i} - 3\hat{j} - \hat{k})

r⃗=−2i^+3j^+k^\vec{r} = -2\hat{i} + 3\hat{j} + \hat{k}

The point is (−2,3,1)(-2, 3, 1)


(B) Direction ratios of the given line → (III) (5, -3, -1)

From the direction vector b⃗=5i^−3j^−k^\vec{b} = 5\hat{i} - 3\hat{j} - \hat{k}

Direction ratios are (5,−3,−1)(5, -3, -1)


(C) Direction cosines of the given line → (I) (535,−335,−135)\left(\frac{5}{\sqrt{35}}, \frac{-3}{\sqrt{35}}, \frac{-1}{\sqrt{35}}\right)

Direction cosines are obtained by dividing direction ratios by their magnitude.

For direction ratios (a,b,c)(a, b, c):

Direction cosines =(aa2+b2+c2,ba2+b2+c2,ca2+b2+c2)= \left(\frac{a}{\sqrt{a^2+b^2+c^2}}, \frac{b}{\sqrt{a^2+b^2+c^2}}, \frac{c}{\sqrt{a^2+b^2+c^2}}\right)

Magnitude =52+(−3)2+(−1)2= \sqrt{5^2 + (-3)^2 + (-1)^2}

=25+9+1= \sqrt{25 + 9 + 1}

=35= \sqrt{35}

Direction cosines =(535,−335,−135)= \left(\frac{5}{\sqrt{35}}, \frac{-3}{\sqrt{35}}, \frac{-1}{\sqrt{35}}\right)


(D) Direction ratios of a line perpendicular to given line → (II) (2, 3, 1)

Two lines are perpendicular when their direction vectors have dot product equal to zero.

For the given line with direction ratios (5,−3,−1)(5, -3, -1) and perpendicular line with (a,b,c)(a, b, c):

5a−3b−c=05a - 3b - c = 0

Checking option (II): (2,3,1)(2, 3, 1)

5(2)−3(3)−1=10−9−1=05(2) - 3(3) - 1 = 10 - 9 - 1 = 0


Final Matching:

(A) → (IV) (−2,3,1)(-2, 3, 1)

(B) → (III) (5,−3,−1)(5, -3, -1)

(C) → (I) (535,−335,−135)\left(\frac{5}{\sqrt{35}}, \frac{-3}{\sqrt{35}}, \frac{-1}{\sqrt{35}}\right)

(D) → (II) (2,3,1)(2, 3, 1)

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