MCQMediumJEE 2023Straight Line Equations

JEE Mathematics 2023 Question with Solution

Let the equations of two adjacent sides of a parallelogram ABCD be 2x3y=232x - 3y = -23 and 5x+4y=235x + 4y = 23. \text{If the equation of its one diagonal AC is 3x+7y=233x + 7y = 23 \text{ and the distance of A from the other diagonal is dd, then 50d250d^2 is equal to:

  • A

    529529

  • B

    625625

  • C

    490490

  • D

    512512

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: The adjacent sides are 2x3y=232x - 3y = -23 and 5x+4y=235x + 4y = 23, and one diagonal is 3x+7y=233x + 7y = 23.

Find: The value of 50d250d^2, where dd is the distance of point AA from the other diagonal.

First, find the vertex AA as the intersection of the two given side lines:

2x3y=232x - 3y = -23 5x+4y=235x + 4y = 23

Solving gives A(4,5)A(-4,5).

Using the diagonal ACAC, the other endpoint is obtained as C(3,2)C(3,2).

Now find the midpoint of diagonal ACAC:

(4+32,5+22)=(12,72)\left( \frac{-4+3}{2}, \frac{5+2}{2} \right) = \left( -\frac{1}{2}, \frac{7}{2} \right)

The diagonals of a parallelogram bisect each other, so diagonal BDBD also passes through (12,72)\left( -\frac{1}{2}, \frac{7}{2} \right). Using the line through the points B(1,7)B(-1,-7) and D(1,2)D(1,2), the equation of diagonal BDBD is obtained as:

7x+y=07x + y = 0

Now use the distance formula from point A(4,5)A(-4,5) to the line 7x+y=07x + y = 0:

d=7(4)+572+12=28+549+1=2350d = \frac{|7(-4)+5|}{\sqrt{7^2+1^2}} = \frac{|-28+5|}{\sqrt{49+1}} = \frac{23}{\sqrt{50}}

Therefore,

50d2=50×(2350)2=50×52950=52950d^2 = 50 \times \left( \frac{23}{\sqrt{50}} \right)^2 = 50 \times \frac{529}{50} = 529

So, the correct option is A.

Common mistakes

  • Finding the distance from point AA to the diagonal ACAC instead of the other diagonal BDBD. This is wrong because the question explicitly asks for the distance from AA to the other diagonal. First determine the equation of BDBD, then apply the point-to-line distance formula.

  • Using an incorrect midpoint property for a parallelogram. The diagonals bisect each other, so the midpoint of ACAC and BDBD must be the same. If this is missed, the equation of BDBD will be wrong.

  • Substituting into the distance formula without taking the absolute value in the numerator. The distance from a point to a line is always non-negative, so use ax1+by1+c/a2+b2|ax_1+by_1+c|/\sqrt{a^2+b^2}.

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