NVAMediumJEE 2023Equation of Plane

JEE Mathematics 2023 Question with Solution

Let λ1,λ2\lambda_1, \lambda_2 be the values of λ\lambda for which the points (1,λ,12)(1, \lambda, \frac{1}{2}) and (2,0,1)(-2, 0, 1) are at equal distance from the plane 2x+3y6z+7=02x + 3y - 6z + 7 = 0. If λ1>λ2\lambda_1 > \lambda_2, then the distance of the point (1λ2,λ2,λ1)(1 - \lambda_2, \lambda_2, \lambda_1) from the line x51=y12=z+72\frac{x-5}{1} = \frac{y-1}{2} = \frac{z+7}{2} is:

Answer

Correct answer:9

Step-by-step solution

Standard Method

Given: The points (1,λ,12)(1, \lambda, \frac{1}{2}) and (2,0,1)(-2, 0, 1) are at equal distance from the plane 2x+3y6z+7=02x + 3y - 6z + 7 = 0.

Find: The distance of the point (1λ2,λ2,λ1)(1 - \lambda_2, \lambda_2, \lambda_1) from the line x51=y12=z+72\frac{x-5}{1} = \frac{y-1}{2} = \frac{z+7}{2}.

Use the distance formula of a point (x1,y1,z1)(x_1, y_1, z_1) from the plane ax+by+cz+d=0ax + by + cz + d = 0:

d=ax1+by1+cz1+da2+b2+c2d = \frac{|ax_1 + by_1 + cz_1 + d|}{\sqrt{a^2 + b^2 + c^2}}

Extracted Working

From the given working, equating the distances of (1,λ,12)(1, \lambda, \frac{1}{2}) and (2,0,1)(-2, 0, 1) from the plane gives

3λ+67=37\left|\frac{3\lambda + 6}{7}\right| = \frac{3}{7}

Solving gives λ1=3\lambda_1 = 3 and λ2=2\lambda_2 = 2.

Hence the required point becomes

(1λ2,λ2,λ1)=(1,2,3)(1 - \lambda_2, \lambda_2, \lambda_1) = (-1, 2, 3)

The line is parameterized, and the distance formula between a point and a line is then used.

Final Answer: The distance is 99.

Common mistakes

  • Using the point-to-plane distance formula without the absolute value is incorrect, because distance cannot be negative. Always use ax1+by1+cz1+d|ax_1 + by_1 + cz_1 + d| in the numerator.

  • Interchanging λ1\lambda_1 and λ2\lambda_2 is incorrect because the condition explicitly states λ1>λ2\lambda_1 > \lambda_2. After solving, assign the larger value to λ1\lambda_1 and the smaller value to λ2\lambda_2.

  • Substituting the point as (1λ1,λ2,λ1)(1-\lambda_1, \lambda_2, \lambda_1) instead of (1λ2,λ2,λ1)(1-\lambda_2, \lambda_2, \lambda_1) changes the coordinates and gives the wrong final distance. Form the point carefully before applying the line-distance formula.

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