Sigma Percentile
JEE Main 2012
LEVELJEE Advanced

Animated Solution for Mathematics - Conic Sections: Statement-1 : An equation of a common tangent to the parabola and the ellipse is Statement-2 : If the line is a common tangent to the parabola and the ellipse , then satisfies

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Visualized Solution

Visualizing the Curves

  • Given Curves:
  • Parabola:
  • Ellipse:
  • Objective: Find the equation of the common tangent and verify the statements.

Standardizing the Ellipse

  • Divide by :
  • Standard form:
  • Comparing, we get and .

Tangent Condition for Ellipse

  • Condition for tangency to ellipse :
  • Substituting and :

Standardizing the Parabola

  • Parabola:
  • Standard form:
  • Comparing:

Tangent Condition for Parabola

  • Condition for tangency to parabola :
  • Substituting :

Equating the Constants

  • For a common tangent, equate the constant terms from (1) and (2):

Squaring Both Sides

  • Squaring both sides to remove the square root:

Rearranging the Equation

  • Multiply by :
  • Rearrange and divide by :

Solving for Slope

  • Factorize the quadratic in :
  • Let , then
  • Since (as is real), we have
  • Therefore,

Verifying Statement 1

  • Substitute into the tangent equation :
  • This matches Statement-1 perfectly.
  • Conclusion: Statement-1 is true, Statement-2 is true, and Statement-2 is the correct explanation for Statement-1.

The Sigma Insight: Equation of Tangent and Normal

Solution Diagram

Analyzing the Setup

Imagine you are standing on a coordinate plane, looking at two distinct geometric entities: a parabola and an ellipse . They seem like strangers, living in different parts of the plane, but they share a secret connection—a common tangent line.
Our mission today is to uncover this line and prove the relationship between these two curves.

Standardizing the Ellipse

First, we must bring our ellipse into the light. The equation is not quite in its standard form.
To reveal its true nature, we divide the entire equation by , yielding:
Now, comparing this to the standard form , we identify and . This is the foundation of our ellipse.

The Parabola's Identity

Next, we turn to the parabola . Comparing this to the standard form , we find , which simplifies beautifully to .
We now have the parameters for both curves.

The Bridge of Tangency

A line is a common tangent if it satisfies the condition of tangency for both curves. For the ellipse, the condition is .
Substituting our values, we get:
For the parabola, the condition is . Substituting , we get:
Now, we bridge the two worlds by equating the constant . By substituting the parabola's into the ellipse's condition, we get:

The Algebraic Triumph

Now, let's solve this. Squaring the left side gives:
So, . Multiplying by and rearranging, we arrive at:
Dividing by , we get the elegant polynomial:
This is the exact condition mentioned in Statement-2! Treating this as a quadratic in , we factor it as .
Since must be positive for a real slope, we find , leading to .
Substituting back into our tangent equation , we get , which simplifies to:
This confirms Statement-1. We have successfully navigated the geometry and the algebra to find the common tangent. The beauty of this problem lies in how two seemingly different curves can be linked by a single, elegant line.

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