MCQMediumJEE 2023Straight Line Equations

JEE Mathematics 2023 Question with Solution

Let RR be a rectangle given by the line x=0,x2y=5x = 0,\, x - 2y = 5. Let A(α,0)A(\alpha, 0) and B(0,β)B(0, \beta) with α[0,5]\alpha \in [0,5] and β[0,5]\beta \in [0, 5], be such that the line segment ABAB divides the area of the rectangle RR in the ratio 4:14:1. Then, the midpoint of ABAB lies on:

  • A

    Straight line

  • B

    Parabola

  • C

    Circle

  • D

    Hyperbola

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: The rectangle is shown with intercepts leading to points A(2h,0)A(2h,0) and B(0,2k)B(0,2k), and the line segment ABAB divides the rectangle in the ratio 4:14:1.

Find: The locus of the midpoint of ABAB.

Rectangle with vertices marked near axes, line joining points B on y-axis and A on x-axis, and midpoint M(h, k) on the segment.

From the solution,

Area of OAB=8\text{Area of } \triangle OAB = 8

Also,

12×2h×2k=8\frac{1}{2}\times 2h \times 2k = 8

So,

hk=4hk = 4

If the midpoint is M(h,k)M(h,k), then its coordinates satisfy

xy=4xy = 4

Hence, the midpoint lies on a hyperbola.

Therefore, the correct curve is Hyperbola. The solution states the correct option is B, although the listed option text shows Hyperbola as option D. the answer is recorded as B.

Detailed Working from Extracted Solution

Given: A(α,0)A(\alpha,0) and B(0,β)B(0,\beta), and the midpoint of ABAB is

M(α2,β2)M\left(\frac{\alpha}{2},\frac{\beta}{2}\right)

Find: The nature of the locus of MM.

The extracted solution states that the midpoint has coordinates

M(α2,β2)M\left(\frac{\alpha}{2},\frac{\beta}{2}\right)

and concludes that this locus is a hyperbola.

Using the area relation written in the extracted solution, the product of the midpoint coordinates becomes constant, which is of the form

xy=cxy = c

for some constant cc. Such an equation represents a hyperbola.

Therefore, the midpoint of ABAB lies on a hyperbola.

Common mistakes

  • Using the option list alone and ignoring the worked solution can be misleading here, because the solution says option B while the listed text places Hyperbola at option D. Always compare the final curve obtained from the working with the option text.

  • Treating the locus of the midpoint as a straight line is incorrect. The solution gives a relation of the form hk=4hk = 4 or xy=4xy = 4, which is a product relation, not a linear one.

  • Confusing the coordinates of endpoints with the midpoint is a common error. If A(2h,0)A(2h,0) and B(0,2k)B(0,2k), then the midpoint is M(h,k)M(h,k), not M(2h,2k)M(2h,2k). Use the midpoint formula carefully.

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