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Parabola question

2007 · Shift 1 · Q39
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  5. /2007 · Shift 1 · Q39

Parabola question

2007 · Shift 1 · Q39

JEE AdvancedMathematicsParabolaMCQ+3 / −1
Consider the circle x2+y2=9{x^2} + {y^2} = 9x2+y2=9 and the parabola y2=8x{y^2} = 8xy2=8x. They intersect at P and Q in the first and the fourth quadrants, respectively. Tangents to the circle at P and Q intersect the x-axis at R and tangents to the parabola at P and Q intersect the x-axis at S.The ratio of the areas of the triangles PQS and PQR is
  1. A
    1 : 2\sqrt22​
  2. B
    1 : 2
  3. C
    1 : 4
  4. D
    1 : 8
View written solutionFree

Correct answer: C

Step-by-step Solution

1. Find the intersection points of the circle and the parabola.

The given equations are:

  • Circle: x2+y2=9{x^2} + {y^2} = 9x2+y2=9 (1)
  • Parabola: y2=8x{y^2} = 8xy2=8x (2)

To find the intersection points, substitute equation (2) into equation (1): x2+8x=9{x^2} + 8x = 9x2+8x=9 x2+8x−9=0{x^2} + 8x - 9 = 0x2+8x−9=0 Factoring the quadratic equation: (x+9)(x−1)=0(x+9)(x-1) = 0(x+9)(x−1)=0 This gives two possible values for x: x=1x = 1x=1 or x=−9x = -9x=−9.

  • If x=−9x = -9x=−9, from equation (2), y2=8(−9)=−72{y^2} = 8(-9) = -72y2=8(−9)=−72. This is not possible for real values of y.
  • If x=1x = 1x=1, from equation (2), y2=8(1)=8{y^2} = 8(1) = 8y2=8(1)=8. This gives y=±8=±22y = \pm \sqrt{8} = \pm 2\sqrt{2}y=±8​=±22​.

The intersection points are (1,22)(1, 2\sqrt{2})(1,22​) and (1,−22)(1, -2\sqrt{2})(1,−22​).

  • Point P is in the first quadrant, so P=(1,22)P = (1, 2\sqrt{2})P=(1,22​).
  • Point Q is in the fourth quadrant, so Q=(1,−22)Q = (1, -2\sqrt{2})Q=(1,−22​).

2. Find the point R (intersection of tangents to the circle with the x-axis).

The equation of the tangent to the circle x2+y2=r2{x^2} + {y^2} = {r^2}x2+y2=r2 at a point (x1,y1)(x_1, y_1)(x1​,y1​) is xx1+yy1=r2x{x_1} + y{y_1} = {r^2}xx1​+yy1​=r2. Here, r2=9{r^2} = 9r2=9.

  • Tangent to the circle at P(1,22)P(1, 2\sqrt{2})P(1,22​): x(1)+y(22)=9  ⟹  x+22y=9x(1) + y(2\sqrt{2}) = 9 \implies x + 2\sqrt{2}y = 9x(1)+y(22​)=9⟹x+22​y=9

To find the intersection point R with the x-axis, set y=0y = 0y=0: x+22(0)=9  ⟹  x=9x + 2\sqrt{2}(0) = 9 \implies x = 9x+22​(0)=9⟹x=9. So, the point R is (9,0)(9, 0)(9,0). Note that the tangent at Q is x−22y=9x - 2\sqrt{2}y = 9x−22​y=9, which also intersects the x-axis at (9,0)(9,0)(9,0).

3. Find the point S (intersection of tangents to the parabola with the x-axis).

The equation of the tangent to the parabola y2=4ax{y^2} = 4axy2=4ax at a point (x1,y1)(x_1, y_1)(x1​,y1​) is yy1=2a(x+x1)y{y_1} = 2a(x + {x_1})yy1​=2a(x+x1​). For the parabola y2=8x{y^2} = 8xy2=8x, we have 4a=84a = 84a=8, so a=2a = 2a=2. The tangent equation is yy1=4(x+x1)y{y_1} = 4(x + {x_1})yy1​=4(x+x1​).

  • Tangent to the parabola at P(1,22)P(1, 2\sqrt{2})P(1,22​): y(22)=4(x+1)y(2\sqrt{2}) = 4(x + 1)y(22​)=4(x+1)

To find the intersection point S with the x-axis, set y=0y = 0y=0: 0=4(x+1)  ⟹  x=−10 = 4(x + 1) \implies x = -10=4(x+1)⟹x=−1. So, the point S is (−1,0)(-1, 0)(−1,0). The tangent at Q, y(−22)=4(x+1)y(-2\sqrt{2}) = 4(x+1)y(−22​)=4(x+1), also intersects the x-axis at (−1,0)(-1,0)(−1,0). A known property for a parabola y2=4axy^2 = 4axy2=4ax is that the tangent at (x1,y1)(x_1, y_1)(x1​,y1​) meets the x-axis at (−x1,0)(-x_1, 0)(−x1​,0). Here x1=1x_1=1x1​=1, so the intersection is at (−1,0)(-1,0)(−1,0), which confirms our result for S.

4. Calculate the ratio of the areas of △PQS\triangle PQS△PQS and △PQR\triangle PQR△PQR.

Both triangles, △PQS\triangle PQS△PQS and △PQR\triangle PQR△PQR, share the same base PQ. The base PQ is a vertical line segment along the line x=1x=1x=1. The length of the base PQ is the distance between P(1,22)P(1, 2\sqrt{2})P(1,22​) and Q(1,−22)Q(1, -2\sqrt{2})Q(1,−22​), which is 22−(−22)=422\sqrt{2} - (-2\sqrt{2}) = 4\sqrt{2}22​−(−22​)=42​.

The area of a triangle is given by 12×base×height\frac{1}{2} \times \text{base} \times \text{height}21​×base×height. Since the base PQ is common, the ratio of the areas is equal to the ratio of their corresponding heights.

  • For △PQS\triangle PQS△PQS, the vertex is S(−1,0)S(-1, 0)S(−1,0). The base PQ lies on the line x=1x=1x=1. The height of △PQS\triangle PQS△PQS is the perpendicular distance from S to the line x=1x=1x=1. HeightS=∣xP−xS∣=∣1−(−1)∣=∣2∣=2_S = |x_P - x_S| = |1 - (-1)| = |2| = 2S​=∣xP​−xS​∣=∣1−(−1)∣=∣2∣=2.

  • For △PQR\triangle PQR△PQR, the vertex is R(9,0)R(9, 0)R(9,0). The base PQ lies on the line x=1x=1x=1. The height of △PQR\triangle PQR△PQR is the perpendicular distance from R to the line x=1x=1x=1. HeightR=∣xR−xP∣=∣9−1∣=∣8∣=8_R = |x_R - x_P| = |9 - 1| = |8| = 8R​=∣xR​−xP​∣=∣9−1∣=∣8∣=8.

Now, let's find the ratio of the areas: Area(△PQS)Area(△PQR)=12×∣PQ∣×HeightS12×∣PQ∣×HeightR=HeightSHeightR\frac{\text{Area}(\triangle PQS)}{\text{Area}(\triangle PQR)} = \frac{\frac{1}{2} \times |PQ| \times \text{Height}_S}{\frac{1}{2} \times |PQ| \times \text{Height}_R} = \frac{\text{Height}_S}{\text{Height}_R}Area(△PQR)Area(△PQS)​=21​×∣PQ∣×HeightR​21​×∣PQ∣×HeightS​​=HeightR​HeightS​​ Area(△PQS)Area(△PQR)=28=14\frac{\text{Area}(\triangle PQS)}{\text{Area}(\triangle PQR)} = \frac{2}{8} = \frac{1}{4}Area(△PQR)Area(△PQS)​=82​=41​

The ratio of the areas of the triangles PQS and PQR is 1 : 4.

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