Sigma Percentile
JEE Advanced 1985
LEVELJEE Main

Animated Solution for Mathematics - Circles: From the origin chords are drawn to the circle . The equation of the locus of the mid-points of these chords is .........

Visualized Solution

Visualize the Circle

  • Given circle:
  • Center: , Radius:

Expand the Equation

  • Expand :
  • Simplify:

Define the Chord and Mid-point

  • Chords are drawn from the origin .
  • Let the mid-point of a chord be .

The Concept

  • Equation of a chord with a given mid-point is:
  • is the tangent expression.
  • is the power of the point.

Constructing

  • For circle and point :

Constructing

  • For circle and point :

Equating

  • Equating :

Apply the Origin Constraint

  • The chord passes through the origin .
  • Substitute into the chord equation.

Atomic Compute - Substitution

Simplify the Equation

  • Rearrange:

Final Locus Equation

  • Replace with to get the locus:

Conclusion and Takeaway

  • The locus is a circle:
  • Center: , Radius:

The Sigma Insight: Equation of Tangent and Normal

Solution Diagram

Analyzing the Setup

The equation describes a circle with its center at and a radius of .
By observing the equation, we see that the origin lies exactly on the boundary of this circle. This serves as the fixed starting point for all chords considered in this problem.

The Master Equation

Expanding the circle equation yields:
This simplifies to the standard form:
Let be the midpoint of a chord originating from . We utilize the chord equation formula , where represents the tangent-like expression at and is the value of the circle equation at .

Constructing the Locus

For the circle , we apply the standard replacement rules: , , and .
This gives us the expression for :
Next, we calculate by substituting the midpoint into the circle equation:
Setting , we obtain the equation of the chord:

Applying the Origin Constraint

Since every chord passes through the origin , the coordinates must satisfy the chord equation. Substituting and into the equation above:
This simplifies to:
Rearranging the terms, we arrive at the final relation:

Final Result

Replacing with the general coordinates , we find the locus of the midpoints:
This result represents a circle with its center at and a radius of . The midpoints of all chords drawn from the origin to the original circle trace out this smaller, perfectly defined geometric path.

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