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Points Equidistant from Two Intersecting Lines

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  • Theorem
CISCE: Class 10

Points Equidistant from Two Intersecting Lines

Statement:
The locus of a point equidistant from two intersecting lines is the pair of angle bisectors of the angles formed by the given lines.

Given:

  • Lines AB and CD intersect at O
  • P is a point inside ∠BOD
  • PM ⟂ OB, PN ⟂ OD
  • PM = PN

To Prove:

  • P lies on the bisector of ∠BOD

Construction:

  • Join OP

Proof:

  1. In ΔOMP and ΔONP, we have
    (i) PM = PN (given)
    (ii) ∠OMP = ∠ONP = 90° (PM ⟂ OB, PN ⟂ OD)
    (iii) OP = OP (common)
  2. ∴ ΔOMP ≅ ΔONP (RHS congruence)

  3. ∴ ∠MOP = ∠PON (c.p.c.t.)

  4. Hence, OP bisects ∠BOD.

Conclusion:
Therefore, P lies on the bisector of ∠BOD.

CISCE: Class 10

Converse: Points Equidistant from Two Intersecting Lines

Statement:
Every point on the angle bisector of two intersecting lines is equidistant from the lines.

Given:

  • Lines AB and CD intersect at O
  • OE is the bisector of ∠BOD
  • P is a point on OE
  • PM ⟂ OB and PN ⟂ OD

To Prove:

PM = PN

Proof:

  1. In ΔOMP and ΔONP, we have
    (i) ∠MOP = ∠PON (OE bisects ∠BOD)
    (ii) ∠OMP = ∠ONP = 90° (PM ⟂ OB, PN ⟂ OD)
    (iii) OP = OP (common)

  2. ∴ ΔOMP ≅ ΔONP (ASA congruence)

  3. ∴ PM = PN (c.p.c.t.)

Conclusion:

Hence, P is equidistant from OB and OD, and therefore from the intersecting lines AB and CD.

Shaalaa.com | Loci Part 3

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