Question:hard

The vector equation of the line whose Cartesian equations are $y = 2$ and $4x - 3z + 5 = 0$ is

Show Hint

Whenever an equation is completely missing a variable (like $x$ missing in $y=2$), the direction ratio for that missing variable is always $0$. This immediately tells you there is no $\hat{j}$ component in the direction vector!
Updated On: Jun 4, 2026
  • $\vec{r} = (2\hat{j} + \hat{k}) + \lambda(3\hat{i} - 4\hat{k})$
  • $\vec{r} = \left(2\hat{j} + \frac{5}{3}\hat{k}\right) + \lambda(3\hat{i} + 4\hat{k})$
  • $\vec{r} = (2\hat{i} + \hat{k}) + \lambda(3\hat{i} + 4\hat{j})$
  • $\vec{r} = \left(2\hat{j} + \frac{5}{3}\hat{k}\right) + \lambda(3\hat{i} - 4\hat{k})$
Show Solution

The Correct Option is B

Solution and Explanation

Step 1: Understand the line.
The line is given by two plane equations $y=2$ and $4x-3z+5=0$. The line is where these two planes cross. We want it as $\vec r=\vec a+\lambda\vec b$.

Step 2: Aim for symmetric form.
We want $\frac{x-x_1}{a}=\frac{y-y_1}{b}=\frac{z-z_1}{c}$. From this we read a point $(x_1,y_1,z_1)$ and a direction $(a,b,c)$.

Step 3: Handle the $y$ part.
Since $y=2$ is fixed, write it as $y-2=0$. The line does not move in the $y$ direction, so its $y$-direction number is $0$.

Step 4: Handle the $x$ and $z$ part.
From $4x-3z+5=0$ we get $4x=3z-5=3\!\left(z-\frac{5}{3}\right)$. Divide by $12$: \[ \frac{x}{3}=\frac{z-\frac{5}{3}}{4}. \]

Step 5: Write the full symmetric form.
\[ \frac{x-0}{3}=\frac{y-2}{0}=\frac{z-\frac{5}{3}}{4}. \]

Step 6: Read the point and direction.
A point on the line is $\left(0,2,\frac{5}{3}\right)$, so $\vec a=2\hat j+\frac{5}{3}\hat k$. The direction numbers are $(3,0,4)$, so $\vec b=3\hat i+4\hat k$.

Step 7: Build the vector equation.
\[ \vec r=\left(2\hat j+\frac{5}{3}\hat k\right)+\lambda\left(3\hat i+4\hat k\right). \] This is option (2).
\[ \boxed{\vec r=\left(2\hat j+\tfrac{5}{3}\hat k\right)+\lambda(3\hat i+4\hat k)} \]
Was this answer helpful?
0