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The direction ratios of the line perpendicular to the lines x−52=y+11−3=z+31\dfrac{x-5}{2} = \dfrac{y+11}{-3} = \dfrac{z+3}{1}and x−71=y+22=z−4−2\dfrac{x-7}{1} = \dfrac{y+2}{2} = \dfrac{z-4}{-2} are proportional to:

Solution

✅ Correct Option: 4

We need to find direction ratios of a line that is perpendicular to BOTH given lines.


The direction ratios are the numbers in the denominators of the symmetric form equation.

Line 1:

x−52=y+11−3=z+31\dfrac{x-5}{2} = \dfrac{y+11}{-3} = \dfrac{z+3}{1}

Direction ratios: (2,−3,1)(2, -3, 1)

Line 2:

x−71=y+22=z−4−2\dfrac{x-7}{1} = \dfrac{y+2}{2} = \dfrac{z-4}{-2}

Direction ratios: (1,2,−2)(1, 2, -2)


When a line is perpendicular to two lines, its direction vector is the cross product of those two lines' direction vectors.

Let d1⃗=(2,−3,1)\vec{d_1} = (2, -3, 1) and d2⃗=(1,2,−2)\vec{d_2} = (1, 2, -2)

The direction ratios we need =d1⃗×d2⃗= \vec{d_1} \times \vec{d_2}

d1⃗×d2⃗=∣i⃗j⃗k⃗2−3112−2∣\vec{d_1} \times \vec{d_2} = \begin{vmatrix} \vec{i} & \vec{j} & \vec{k} \\ 2 & -3 & 1 \\ 1 & 2 & -2 \end{vmatrix}

For i⃗\vec{i} component:

i⃗[(−3)(−2)−(1)(2)]=i⃗[6−2]=4i⃗\vec{i}[(-3)(-2) - (1)(2)] = \vec{i}[6 - 2] = 4\vec{i}

For j⃗\vec{j} component (note the negative sign):

−j⃗[(2)(−2)−(1)(1)]=−j⃗[−4−1]=−j⃗[−5]=5j⃗-\vec{j}[(2)(-2) - (1)(1)] = -\vec{j}[-4 - 1] = -\vec{j}[-5] = 5\vec{j}

For k⃗\vec{k} component:

k⃗[(2)(2)−(−3)(1)]=k⃗[4+3]=7k⃗\vec{k}[(2)(2) - (-3)(1)] = \vec{k}[4 + 3] = 7\vec{k}


Combining all components: (4,5,7)(4, 5, 7)

Quick verification using dot products:

(4,5,7)⋅(2,−3,1)=8−15+7=0(4, 5, 7) \cdot (2, -3, 1) = 8 - 15 + 7 = 0 ✓

(4,5,7)⋅(1,2,−2)=4+10−14=0(4, 5, 7) \cdot (1, 2, -2) = 4 + 10 - 14 = 0 ✓


The direction ratios are proportional to (4,5,7)(4, 5, 7)

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