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JEE Main 2023
3D Geometry
3D Geometry
Hard

Question

The line, that is coplanar to the line x+33=y11=z55\frac{x+3}{-3}=\frac{y-1}{1}=\frac{z-5}{5}, is :

Options

Solution

Here's a detailed and well-structured solution to the problem, adhering to the specified format and rules.


  1. Key Concepts and Formulas

    • Equation of a Line in Cartesian Form: A line passing through a point P1(x1,y1,z1)P_1(x_1, y_1, z_1) with direction ratios (a1,b1,c1)(a_1, b_1, c_1) is given by: xx1a1=yy1b1=zz1c1\frac{x - x_1}{a_1} = \frac{y - y_1}{b_1} = \frac{z - z_1}{c_1}
    • Condition for Coplanarity of Two Lines: Two lines, L1L_1 and L2L_2, are coplanar if they lie in the same plane. This occurs if they are parallel or if they intersect. Let L1L_1 pass through P1(x1,y1,z1)P_1(x_1, y_1, z_1) with direction ratios (a1,b1,c1)(a_1, b_1, c_1). Let L2L_2 pass through P2(x2,y2,z2)P_2(x_2, y_2, z_2) with direction ratios (a2,b2,c2)(a_2, b_2, c_2). The lines L1L_1 and L2L_2 are coplanar if and only if the scalar triple product of the vector P1P2\vec{P_1P_2} and their direction vectors d1\vec{d_1} and d2\vec{d_2} is zero. This is expressed using the determinant: x2x1y2y1z2z1a1b1c1a2b2c2=0\begin{vmatrix} x_2-x_1 & y_2-y_1 & z_2-z_1 \\ a_1 & b_1 & c_1 \\ a_2 & b_2 & c_2 \end{vmatrix} = 0
    • Interpretation of the Determinant: The first row represents the vector connecting a point on L1L_1 to a point on L2L_2. The second and third rows represent the direction vectors of L1L_1 and L2L_2, respectively. If these three vectors are coplanar, the lines themselves must be coplanar.
  2. Step-by-Step Solution

    Step 1: Identify the point and direction ratios for the given line (L1L_1). The given line is: L1:x+33=y11=z55L_1: \frac{x+3}{-3}=\frac{y-1}{1}=\frac{z-5}{5} Comparing this with the standard form xx1a1=yy1b1=zz1c1\frac{x-x_1}{a_1} = \frac{y-y_1}{b_1} = \frac{z-z_1}{c_1}:

    • Point P1(x1,y1,z1)=(3,1,5)P_1(x_1, y_1, z_1) = (-3, 1, 5).
    • Direction ratios d1(a1,b1,c1)=(3,1,5)\vec{d_1}(a_1, b_1, c_1) = (-3, 1, 5).

    Step 2: Test each option for coplanarity with L1L_1. For each option, we'll extract a point P2(x2,y2,z2)P_2(x_2, y_2, z_2) and direction ratios d2(a2,b2,c2)\vec{d_2}(a_2, b_2, c_2), then calculate the determinant.

    Option (A): The line is LA:x+11=y22=z54L_A: \frac{x+1}{-1}=\frac{y-2}{2}=\frac{z-5}{4}

    • Point PA(1,2,5)P_A(-1, 2, 5).
    • Direction ratios dA(1,2,4)\vec{d_A}(-1, 2, 4).
    • Vector P1PA=(x2x1,y2y1,z2z1)=(1(3),21,55)=(2,1,0)\vec{P_1P_A} = (x_2-x_1, y_2-y_1, z_2-z_1) = (-1 - (-3), 2 - 1, 5 - 5) = (2, 1, 0).
    • Calculate the determinant: 210315124\begin{vmatrix} 2 & 1 & 0 \\ -3 & 1 & 5 \\ -1 & 2 & 4 \end{vmatrix} Expanding along the first row: = 2 \left| \begin{matrix} 1 & 5 \\ 2 & 4 \end{vmatrix} \right| - 1 \left| \begin{matrix} -3 & 5 \\ -1 & 4 \end{vmatrix} \right| + 0 \left| \begin{matrix} -3 & 1 \\ -1 & 2 \end{vmatrix} \right| =2((1)(4)(5)(2))1((3)(4)(5)(1))+0= 2((1)(4) - (5)(2)) - 1((-3)(4) - (5)(-1)) + 0 =2(410)1(12+5)= 2(4 - 10) - 1(-12 + 5) =2(6)1(7)= 2(-6) - 1(-7) =12+7=5= -12 + 7 = -5 Since the determinant is 50-5 \neq 0, line LAL_A is not coplanar with L1L_1.

    Option (B): The line is LB:x+11=y22=z55L_B: \frac{x+1}{-1}=\frac{y-2}{2}=\frac{z-5}{5}

    • Point PB(1,2,5)P_B(-1, 2, 5).
    • Direction ratios dB(1,2,5)\vec{d_B}(-1, 2, 5).
    • Vector P1PB=(1(3),21,55)=(2,1,0)\vec{P_1P_B} = (-1 - (-3), 2 - 1, 5 - 5) = (2, 1, 0).
    • Calculate the determinant: 210315125\begin{vmatrix} 2 & 1 & 0 \\ -3 & 1 & 5 \\ -1 & 2 & 5 \end{vmatrix} Expanding along the first row: = 2 \left| \begin{matrix} 1 & 5 \\ 2 & 5 \end{vmatrix} \right| - 1 \left| \begin{matrix} -3 & 5 \\ -1 & 5 \end{vmatrix} \right| + 0 \left| \begin{matrix} -3 & 1 \\ -1 & 2 \end{vmatrix} \right| =2((1)(5)(5)(2))1((3)(5)(5)(1))+0= 2((1)(5) - (5)(2)) - 1((-3)(5) - (5)(-1)) + 0 =2(510)1(15+5)= 2(5 - 10) - 1(-15 + 5) =2(5)1(10)= 2(-5) - 1(-10) =10+10=0= -10 + 10 = 0 Since the determinant is 00, line LBL_B is coplanar with L1L_1.

    Option (C): The line is LC:x11=y22=z55L_C: \frac{x-1}{-1}=\frac{y-2}{2}=\frac{z-5}{5}

    • Point PC(1,2,5)P_C(1, 2, 5).
    • Direction ratios dC(1,2,5)\vec{d_C}(-1, 2, 5).
    • Vector P1PC=(1(3),21,55)=(4,1,0)\vec{P_1P_C} = (1 - (-3), 2 - 1, 5 - 5) = (4, 1, 0).
    • Calculate the determinant: 410315125\begin{vmatrix} 4 & 1 & 0 \\ -3 & 1 & 5 \\ -1 & 2 & 5 \end{vmatrix} Expanding along the first row: =4((1)(5)(5)(2))1((3)(5)(5)(1))+0= 4((1)(5) - (5)(2)) - 1((-3)(5) - (5)(-1)) + 0 =4(510)1(15+5)= 4(5 - 10) - 1(-15 + 5) =4(5)1(10)= 4(-5) - 1(-10) =20+10=10= -20 + 10 = -10 Since the determinant is 100-10 \neq 0, line LCL_C is not coplanar with L1L_1.

    Option (D): The line is LD:x+11=y22=z55L_D: \frac{x+1}{1}=\frac{y-2}{2}=\frac{z-5}{5}

    • Point PD(1,2,5)P_D(-1, 2, 5).
    • Direction ratios dD(1,2,5)\vec{d_D}(1, 2, 5).
    • Vector P1PD=(1(3),21,55)=(2,1,0)\vec{P_1P_D} = (-1 - (-3), 2 - 1, 5 - 5) = (2, 1, 0).
    • Calculate the determinant: 210315125\begin{vmatrix} 2 & 1 & 0 \\ -3 & 1 & 5 \\ 1 & 2 & 5 \end{vmatrix} Expanding along the first row: =2((1)(5)(5)(2))1((3)(5)(5)(1))+0= 2((1)(5) - (5)(2)) - 1((-3)(5) - (5)(1)) + 0 =2(510)1(155)= 2(5 - 10) - 1(-15 - 5) =2(5)1(20)= 2(-5) - 1(-20) =10+20=10= -10 + 20 = 10 Since the determinant is 10010 \neq 0, line LDL_D is not coplanar with L1L_1.
  3. Common Mistakes & Tips

    • Sign Errors in Point Extraction: Always remember that the standard form is xx1a1\frac{x-x_1}{a_1}. So, if you have (x+3)(x+3), x1x_1 is 3-3, not 33.
    • Determinant Calculation: Be meticulous with signs during determinant expansion, especially when subtracting terms. A common mistake is (adbc)-(a \cdot d - b \cdot c) becoming adbc-ad - bc instead of ad+bc-ad + bc.
    • Parallel Lines: If two lines are parallel, their direction ratios will be proportional, meaning (a1,b1,c1)=k(a2,b2,c2)(a_1, b_1, c_1) = k(a_2, b_2, c_2) for some scalar kk. In this case, the second and third rows of the coplanarity determinant will be proportional, making the determinant automatically zero. Thus, parallel lines are always coplanar. Check for this shortcut first if the direction vectors seem related. (In this problem, no options were parallel to L1L_1).
  4. Summary

    To determine which line is coplanar with the given line, we applied the condition for coplanarity, which involves setting a specific determinant to zero. We extracted a point and direction vector from the given line and then did the same for each option. For each option, we formed a 3×33 \times 3 determinant using the vector connecting the two points and their respective direction vectors. A determinant value of zero indicates coplanarity. Our calculations showed that only Option (B) satisfied this condition, yielding a determinant of 0.

The final answer is A\boxed{A}.

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