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Straight Lines and Pair of Straight Lines question

2025 · Shift 2 · Q20
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Straight Lines and Pair of Straight Lines question

2025 · Shift 2 · Q20

JEE AdvancedMathematicsStraight Lines and Pair of Straight LinesMCQ+3 / −1
Let S denote the locus of the point of intersection of the pair of lines 4x−3y=12α4x - 3y = 12\alpha4x−3y=12α, 4αx+3αy=124\alpha x + 3\alpha y = 124αx+3αy=12, where α\alphaα varies over the set of non-zero real numbers. Let T be the tangent to S passing through the points (p,0)(p, 0)(p,0) and (0,q)(0, q)(0,q), q>0q \gt 0q>0, and parallel to the line 4x−32y=04x - \frac{3}{\sqrt{2}} y = 04x−2​3​y=0. Then the value of pqpqpq is :
  1. A
    −62-6\sqrt{2}−62​
  2. B
    −32-3\sqrt{2}−32​
  3. C
    −92-9\sqrt{2}−92​
  4. D
    −122-12\sqrt{2}−122​
View written solutionFree

Correct answer: A

Step 1: Find the equation of the locus S

The point of intersection (x,y)(x, y)(x,y) is given by the solution of the two linear equations:

  1. 4x−3y=12α4x - 3y = 12\alpha4x−3y=12α
  2. 4αx+3αy=124\alpha x + 3\alpha y = 124αx+3αy=12

Our goal is to eliminate the parameter α\alphaα. From equation (1), we can express α\alphaα in terms of xxx and yyy: α=4x−3y12\alpha = \frac{4x - 3y}{12}α=124x−3y​ Since α\alphaα is a non-zero real number, 4x−3y≠04x - 3y \neq 04x−3y=0.

From equation (2), we can also express α\alphaα: α(4x+3y)=12  ⟹  α=124x+3y\alpha (4x + 3y) = 12 \implies \alpha = \frac{12}{4x + 3y}α(4x+3y)=12⟹α=4x+3y12​ Since α\alphaα is non-zero, 4x+3y≠04x + 3y \neq 04x+3y=0.

Now, we equate the two expressions for α\alphaα: 4x−3y12=124x+3y\frac{4x - 3y}{12} = \frac{12}{4x + 3y}124x−3y​=4x+3y12​ Cross-multiplying gives: (4x−3y)(4x+3y)=12×12(4x - 3y)(4x + 3y) = 12 \times 12(4x−3y)(4x+3y)=12×12 Using the difference of squares formula (a−b)(a+b)=a2−b2(a-b)(a+b) = a^2 - b^2(a−b)(a+b)=a2−b2: (4x)2−(3y)2=144(4x)^2 - (3y)^2 = 144(4x)2−(3y)2=144 16x2−9y2=14416x^2 - 9y^2 = 14416x2−9y2=144 To write this in the standard form of a conic section, we divide the entire equation by 144: 16x2144−9y2144=1\frac{16x^2}{144} - \frac{9y^2}{144} = 114416x2​−1449y2​=1 x29−y216=1\frac{x^2}{9} - \frac{y^2}{16} = 19x2​−16y2​=1 This is the equation of a hyperbola. So, the locus S is a hyperbola with a2=9a^2 = 9a2=9 and b2=16b^2 = 16b2=16.

Step 2: Find the equation of the tangent T

The tangent T is parallel to the line 4x−32y=04x - \frac{3}{\sqrt{2}} y = 04x−2​3​y=0. The slope of this line is given by m=−coefficient of xcoefficient of ym = -\frac{\text{coefficient of } x}{\text{coefficient of } y}m=−coefficient of ycoefficient of x​. m=−4−3/2=423m = -\frac{4}{-3/\sqrt{2}} = \frac{4\sqrt{2}}{3}m=−−3/2​4​=342​​ Since T is parallel to this line, the slope of tangent T is mT=423m_T = \frac{4\sqrt{2}}{3}mT​=342​​.

The equation of a tangent to the hyperbola x2a2−y2b2=1\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1a2x2​−b2y2​=1 with slope mmm is given by the formula: y=mx±a2m2−b2y = mx \pm \sqrt{a^2m^2 - b^2}y=mx±a2m2−b2​ Substituting the values a2=9a^2 = 9a2=9, b2=16b^2 = 16b2=16, and m=423m = \frac{4\sqrt{2}}{3}m=342​​: a2m2−b2=9(423)2−16a^2m^2 - b^2 = 9 \left( \frac{4\sqrt{2}}{3} \right)^2 - 16a2m2−b2=9(342​​)2−16 =9(16×29)−16= 9 \left( \frac{16 \times 2}{9} \right) - 16=9(916×2​)−16 =32−16=16= 32 - 16 = 16=32−16=16 So, the condition for tangency is satisfied. The equations of the two possible tangents are: y=423x±16y = \frac{4\sqrt{2}}{3}x \pm \sqrt{16}y=342​​x±16​ y=423x±4y = \frac{4\sqrt{2}}{3}x \pm 4y=342​​x±4

Step 3: Identify the correct tangent and find p and q

The tangent T passes through the points (p,0)(p, 0)(p,0) and (0,q)(0, q)(0,q). This means ppp is the x-intercept and qqq is the y-intercept of the tangent line.

From the two possible tangent equations, the y-intercepts (when x=0x=0x=0) are y=4y = 4y=4 and y=−4y = -4y=−4. The problem states that q>0q > 0q>0. Therefore, we must choose the tangent with the positive y-intercept. q=4q = 4q=4 The equation of the tangent T is: y=423x+4y = \frac{4\sqrt{2}}{3}x + 4y=342​​x+4 To find the x-intercept ppp, we set y=0y=0y=0 in the equation of T: 0=423x+40 = \frac{4\sqrt{2}}{3}x + 40=342​​x+4 423x=−4\frac{4\sqrt{2}}{3}x = -4342​​x=−4 x=−4×342=−32x = -4 \times \frac{3}{4\sqrt{2}} = -\frac{3}{\sqrt{2}}x=−4×42​3​=−2​3​ Rationalizing the denominator: p=x=−322p = x = -\frac{3\sqrt{2}}{2}p=x=−232​​

Step 4: Calculate the value of pq

We have found p=−322p = -\frac{3\sqrt{2}}{2}p=−232​​ and q=4q = 4q=4. Now we compute their product: pq=(−322)×4pq = \left( -\frac{3\sqrt{2}}{2} \right) \times 4pq=(−232​​)×4 pq=−32×2pq = -3\sqrt{2} \times 2pq=−32​×2 pq=−62pq = -6\sqrt{2}pq=−62​ This corresponds to option A.

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