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Hyperbola question

2020 · Shift 2 · Q26
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  5. /2020 · Shift 2 · Q26

Hyperbola question

2020 · Shift 2 · Q26

JEE AdvancedMathematicsHyperbolaMultiple correct+4 / −2
Let a and b be positive real numbers such that a > 1 and b < a. Let P be a point in the first quadrant that lies on the hyperbola x2a2−y2b2=1{{{x^2}} \over {{a^2}}} - {{{y^2}} \over {{b^2}}} = 1a2x2​−b2y2​=1. Suppose the tangent to the hyperbola at P passes through the point (1, 0), and suppose the normal to the hyperbola at P cuts off equal intercepts on the coordinate axes. Let Δ\DeltaΔ denote the area of the triangle formed by the tangent at P, the normal at P and the X-axis. If e denotes the eccentricity of the hyperbola, then which of the following statements is/are TRUE?
  1. A
    1<e<21 \lt e \lt \sqrt 21<e<2​
  2. B
    2<e<2\sqrt 2 \lt e \lt 22​<e<2
  3. C
    Δ=a4\Delta = {a^4}Δ=a4
  4. D
    Δ=b4\Delta = {b^4}Δ=b4
View written solutionFree

Correct answer: A, D

Step-by-Step Solution

  1. Coordinates of Point P and Tangent Information Let the point P on the hyperbola x2a2−y2b2=1\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1a2x2​−b2y2​=1 in the first quadrant be P(x1,y1)P(x_1, y_1)P(x1​,y1​), where x1>0x_1 > 0x1​>0 and y1>0y_1 > 0y1​>0. The equation of the tangent to the hyperbola at P(x1,y1)P(x_1, y_1)P(x1​,y1​) is given by: xx1a2−yy1b2=1\frac{xx_1}{a^2} - \frac{yy_1}{b^2} = 1a2xx1​​−b2yy1​​=1 We are given that this tangent passes through the point (1, 0). Substituting x=1x=1x=1 and y=0y=0y=0 into the tangent equation: 1⋅x1a2−0⋅y1b2=1\frac{1 \cdot x_1}{a^2} - \frac{0 \cdot y_1}{b^2} = 1a21⋅x1​​−b20⋅y1​​=1 x1a2=1  ⟹  x1=a2\frac{x_1}{a^2} = 1 \implies x_1 = a^2a2x1​​=1⟹x1​=a2

  2. Normal Information The equation of the normal to the hyperbola at P(x1,y1)P(x_1, y_1)P(x1​,y1​) is: a2xx1+b2yy1=a2+b2\frac{a^2x}{x_1} + \frac{b^2y}{y_1} = a^2 + b^2x1​a2x​+y1​b2y​=a2+b2 The normal cuts off equal intercepts on the coordinate axes. Let's find the intercepts:

    • x-intercept (set y=0y=0y=0): a2xx1=a2+b2  ⟹  x=x1(a2+b2)a2\frac{a^2x}{x_1} = a^2 + b^2 \implies x = \frac{x_1(a^2 + b^2)}{a^2}x1​a2x​=a2+b2⟹x=a2x1​(a2+b2)​
    • y-intercept (set x=0x=0x=0): b2yy1=a2+b2  ⟹  y=y1(a2+b2)b2\frac{b^2y}{y_1} = a^2 + b^2 \implies y = \frac{y_1(a^2 + b^2)}{b^2}y1​b2y​=a2+b2⟹y=b2y1​(a2+b2)​ Since the intercepts are equal: x1(a2+b2)a2=y1(a2+b2)b2\frac{x_1(a^2 + b^2)}{a^2} = \frac{y_1(a^2 + b^2)}{b^2}a2x1​(a2+b2)​=b2y1​(a2+b2)​ As a,b>0a, b > 0a,b>0, a2+b2≠0a^2 + b^2 \neq 0a2+b2=0, so we can divide both sides by a2+b2a^2 + b^2a2+b2: x1a2=y1b2\frac{x_1}{a^2} = \frac{y_1}{b^2}a2x1​​=b2y1​​
  3. Determining the Coordinates of P and the Relation between a and b From step 1, we have x1=a2x_1 = a^2x1​=a2. Substituting this into the relation from step 2: a2a2=y1b2  ⟹  1=y1b2  ⟹  y1=b2\frac{a^2}{a^2} = \frac{y_1}{b^2} \implies 1 = \frac{y_1}{b^2} \implies y_1 = b^2a2a2​=b2y1​​⟹1=b2y1​​⟹y1​=b2 So, the point P is (a2,b2)(a^2, b^2)(a2,b2). Since P lies on the hyperbola x2a2−y2b2=1\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1a2x2​−b2y2​=1, its coordinates must satisfy the equation: (a2)2a2−(b2)2b2=1\frac{(a^2)^2}{a^2} - \frac{(b^2)^2}{b^2} = 1a2(a2)2​−b2(b2)2​=1 a2−b2=1a^2 - b^2 = 1a2−b2=1

  4. Calculating the Eccentricity (e) The eccentricity of a hyperbola is given by the relation b2=a2(e2−1)b^2 = a^2(e^2 - 1)b2=a2(e2−1). From our derived relation, b2=a2−1b^2 = a^2 - 1b2=a2−1. Substituting this into the eccentricity formula: a2−1=a2(e2−1)a^2 - 1 = a^2(e^2 - 1)a2−1=a2(e2−1) a2−1=a2e2−a2a^2 - 1 = a^2e^2 - a^2a2−1=a2e2−a2 2a2−1=a2e22a^2 - 1 = a^2e^22a2−1=a2e2 e2=2a2−1a2=2−1a2e^2 = \frac{2a^2 - 1}{a^2} = 2 - \frac{1}{a^2}e2=a22a2−1​=2−a21​ We are given that a>1a > 1a>1, which implies a2>1a^2 > 1a2>1. Therefore, 0<1a2<10 < \frac{1}{a^2} < 10<a21​<1. This leads to the inequality for e2e^2e2: 2−1<2−1a2<2−02 - 1 < 2 - \frac{1}{a^2} < 2 - 02−1<2−a21​<2−0 1<e2<21 < e^2 < 21<e2<2 Since eccentricity e>1e > 1e>1 for a hyperbola, we take the positive square root: 1<e<21 < e < \sqrt{2}1<e<2​

    • This means statement A is TRUE.
    • This means statement B is FALSE.
  5. Calculating the Area of the Triangle (Δ\DeltaΔ) The triangle is formed by the tangent at P, the normal at P, and the X-axis. The vertices of this triangle are the intersection points of these three lines.

    • Vertex 1 (Intersection of Tangent and Normal): This is the point P(x1,y1)=(a2,b2)P(x_1, y_1) = (a^2, b^2)P(x1​,y1​)=(a2,b2).
    • Vertex 2 (Intersection of Tangent and X-axis): The tangent equation at P(a2,b2)P(a^2, b^2)P(a2,b2) is x(a2)a2−y(b2)b2=1  ⟹  x−y=1 \frac{x(a^2)}{a^2} - \frac{y(b^2)}{b^2} = 1 \implies x - y = 1a2x(a2)​−b2y(b2)​=1⟹x−y=1. Setting y=0y=0y=0, we get x=1x=1x=1. So, the vertex is T(1,0)T(1, 0)T(1,0).
    • Vertex 3 (Intersection of Normal and X-axis): The normal equation at P(a2,b2)P(a^2, b^2)P(a2,b2) is a2xa2+b2yb2=a2+b2  ⟹  x+y=a2+b2 \frac{a^2x}{a^2} + \frac{b^2y}{b^2} = a^2 + b^2 \implies x + y = a^2 + b^2a2a2x​+b2b2y​=a2+b2⟹x+y=a2+b2. Setting y=0y=0y=0, we get x=a2+b2x = a^2 + b^2x=a2+b2. So, the vertex is N(a2+b2,0)N(a^2 + b^2, 0)N(a2+b2,0).

    The triangle has vertices P(a2,b2)P(a^2, b^2)P(a2,b2), T(1,0)T(1, 0)T(1,0), and N(a2+b2,0)N(a^2 + b^2, 0)N(a2+b2,0). The base of the triangle lies on the X-axis, with length equal to the distance between T and N: Base=∣(a2+b2)−1∣=a2+b2−1\text{Base} = |(a^2 + b^2) - 1| = a^2 + b^2 - 1Base=∣(a2+b2)−1∣=a2+b2−1 (since a>1,b>0a > 1, b > 0a>1,b>0, a2+b2−1>0a^2+b^2-1 > 0a2+b2−1>0) The height of the triangle is the y-coordinate of P, which is y1=b2y_1 = b^2y1​=b2. The area Δ\DeltaΔ is: Δ=12×Base×Height=12(a2+b2−1)(b2)\Delta = \frac{1}{2} \times \text{Base} \times \text{Height} = \frac{1}{2} (a^2 + b^2 - 1)(b^2)Δ=21​×Base×Height=21​(a2+b2−1)(b2) Using the relation a2−b2=1  ⟹  a2−1=b2a^2 - b^2 = 1 \implies a^2 - 1 = b^2a2−b2=1⟹a2−1=b2, we substitute this into the area formula: Δ=12((a2−1)+b2)(b2)=12(b2+b2)(b2)=12(2b2)(b2)=b4\Delta = \frac{1}{2} ((a^2 - 1) + b^2)(b^2) = \frac{1}{2} (b^2 + b^2)(b^2) = \frac{1}{2} (2b^2)(b^2) = b^4Δ=21​((a2−1)+b2)(b2)=21​(b2+b2)(b2)=21​(2b2)(b2)=b4

    • This means statement D is TRUE.
    • This means statement C is FALSE.

Conclusion

Based on the analysis, statements A and D are true.

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