MCQMediumJEE 2025Conic Sections (Parabola, Ellipse, Hyperbola)

JEE Mathematics 2025 Question with Solution

A rod of length eight units moves such that its ends A and B always lie on the lines xy+2=0x - y + 2 = 0 and y+2=0y + 2 = 0, respectively. If the locus of the point PP, that divides the rod ABAB internally in the ratio 2:12:1, is 9(x2+αy2+βxy+γx+28y)76=0,9(x^2 + \alpha y^2 + \beta xy + \gamma x + 28 y) - 76 = 0, then αβγ\alpha - \beta - \gamma is equal to:

  • A

    2424

  • B

    2323

  • C

    2121

  • D

    2222

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: End AA lies on xy+2=0x-y+2=0 and end BB lies on y+2=0y+2=0. The rod has fixed length 88 units. Point P(x,y)P(x,y) divides ABAB internally in the ratio 2:12:1.

Find: αβγ\alpha-\beta-\gamma from the locus of PP.

Take

A=(x1,x1+2),B=(x2,2)A=(x_1,x_1+2), \qquad B=(x_2,-2)

since AA lies on xy+2=0x-y+2=0 and BB lies on y=2y=-2.

Using the length of the rod,

(x2x1)2+((2)(x1+2))2=8\sqrt{(x_2-x_1)^2+((-2)-(x_1+2))^2}=8

so

(x2x1)2+(x1+4)2=64(x_2-x_1)^2+(x_1+4)^2=64

Since PP divides ABAB internally in the ratio 2:12:1,

P=(2x2+x13,2(2)+(x1+2)3)P=\left(\frac{2x_2+x_1}{3},\frac{2(-2)+(x_1+2)}{3}\right)

Hence,

x=2x2+x13,y=x123x=\frac{2x_2+x_1}{3}, \qquad y=\frac{x_1-2}{3}

From y=x123y=\frac{x_1-2}{3}, we get

x1=3y+2x_1=3y+2

And from x=2x2+x13x=\frac{2x_2+x_1}{3},

2x2=3xx1=3x3y22x_2=3x-x_1=3x-3y-2

so

x2=3x3y22x_2=\frac{3x-3y-2}{2}

Now,

x2x1=3x3y22(3y+2)=3x9y62=32(x3y2)x_2-x_1=\frac{3x-3y-2}{2}-(3y+2)=\frac{3x-9y-6}{2}=\frac{3}{2}(x-3y-2)

and

x1+4=3y+6=3(y+2)x_1+4=3y+6=3(y+2)

Substitute in the length condition:

[32(x3y2)]2+[3(y+2)]2=64\left[\frac{3}{2}(x-3y-2)\right]^2+[3(y+2)]^2=64 94(x3y2)2+9(y+2)2=64\frac{9}{4}(x-3y-2)^2+9(y+2)^2=64

Multiply by 44:

9(x3y2)2+36(y+2)2=2569(x-3y-2)^2+36(y+2)^2=256

Divide by 44 form equivalently to match the given locus after expansion.

Expanding,

9(x2+9y2+46xy4x+12y)+36(y2+4y+4)=2569(x^2+9y^2+4-6xy-4x+12y)+36(y^2+4y+4)=256 9x2+81y2+3654xy36x+108y+36y2+144y+144=2569x^2+81y^2+36-54xy-36x+108y+36y^2+144y+144=256 9x2+117y254xy36x+252y76=09x^2+117y^2-54xy-36x+252y-76=0

Factor out 99 from the required part:

9(x2+13y26xy4x+28y)76=09\left(x^2+13y^2-6xy-4x+28y\right)-76=0

Comparing with

9(x2+αy2+βxy+γx+28y)76=09(x^2+\alpha y^2+\beta xy+\gamma x+28y)-76=0

we get

α=13,β=6,γ=4\alpha=13, \qquad \beta=-6, \qquad \gamma=-4

Therefore,

αβγ=13(6)(4)=13+6+4=23\alpha-\beta-\gamma=13-(-6)-(-4)=13+6+4=23

So, the correct option is B.

Using section formula and elimination

Given: AA lies on xy+2=0x-y+2=0, BB lies on y=2y=-2, and AB=8AB=8.

Find: The value of αβγ\alpha-\beta-\gamma.

Write the moving points as

A=(t,t+2),B=(s,2)A=(t,t+2), \qquad B=(s,-2)

Then

AB2=(st)2+(t+4)2=64AB^2=(s-t)^2+(t+4)^2=64

If P(x,y)P(x,y) divides ABAB in the ratio 2:12:1, then

x=2s+t3,y=t23x=\frac{2s+t}{3}, \qquad y=\frac{t-2}{3}

So

t=3y+2,s=3x3y22t=3y+2, \qquad s=\frac{3x-3y-2}{2}

Substitute these into

(st)2+(t+4)2=64(s-t)^2+(t+4)^2=64

This gives

(3x3y22(3y+2))2+(3y+6)2=64\left(\frac{3x-3y-2}{2}-(3y+2)\right)^2+(3y+6)^2=64 (3x9y62)2+(3y+6)2=64\left(\frac{3x-9y-6}{2}\right)^2+(3y+6)^2=64

Now simplify:

94(x3y2)2+9(y+2)2=64\frac{9}{4}(x-3y-2)^2+9(y+2)^2=64 9(x3y2)2+36(y+2)2=2569(x-3y-2)^2+36(y+2)^2=256

Expanding and collecting terms,

9x2+117y254xy36x+252y76=09x^2+117y^2-54xy-36x+252y-76=0

Hence,

9(x2+13y26xy4x+28y)76=09(x^2+13y^2-6xy-4x+28y)-76=0

Therefore,

α=13,β=6,γ=4\alpha=13, \quad \beta=-6, \quad \gamma=-4

and

αβγ=23\alpha-\beta-\gamma=23

Hence the correct option is B.

Common mistakes

  • Using the wrong section formula. If PP divides ABAB internally in the ratio 2:12:1, the coordinates are weighted toward the opposite endpoint. Use P=(2x2+x13,2y2+y13)P=\left(\frac{2x_2+x_1}{3},\frac{2y_2+y_1}{3}\right) consistently.

  • Writing the line condition for point AA incorrectly. From xy+2=0x-y+2=0, we must get y=x+2y=x+2, not y=x2y=x-2. A sign error here changes every coefficient in the locus.

  • Making an error in the distance formula. The vertical difference between A(x1,x1+2)A(x_1,x_1+2) and B(x2,2)B(x_2,-2) is 2(x1+2)=(x1+4)-2-(x_1+2)=-(x_1+4), whose square is (x1+4)2(x_1+4)^2, not (x2+4)2(x_2+4)^2.

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