CBSE Class 9 Mathematics Question 2 of 12

Circles — Question 2

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Question 2

If two equal chords of a circle intersect within the circle, prove that the segments of one chord are equal to corresponding segments of the other chord.

Answer

Let AB and CD be the two equal chords (AB = CD = a). Let the chords intersect at point P. Join OP.

Draw OM and ON perpendicular to chords AB and CD respectively.

If two equal chords of a circle intersect within the circle, prove that the segments of one chord are equal to corresponding segments of the other chord. NCERT Class 9 Mathematics CBSE Solutions.

We know that,

Perpendicular from center bisects the chord.

∴ AM = MB = AB2\dfrac{AB}{2} and CN = DN = CD2\dfrac{CD}{2}

Since, AB = CD.

∴ AM = MB = CN = DN = x(let) .....(1)

In ∆ OMP and ∆ ONP,

⇒ ∠M = ∠N (Both equal to 90°)

⇒ OP = OP (Common side)

⇒ OM = ON (Equal chords are equidistant from the center.)

∴ ∆ OMP ≅ ∆ ONP (By R.H.S congruence rule)

We know that,

Corresponding parts of congruent triangles are equal.

∴ MP = NP = y(let) (By C.P.C.T.) .....(2)

From figure,

⇒ CP = CN - NP = x - y and PB = MB - MP = x - y

∴ CP = PB.

From figure,

DP = CD - CP = a - (x - y) and AP = AB - BP = a - (x - y)

∴ AP = PD.

Hence, proved that if two equal chords of a circle intersect within the circle, the segments of one chord are equal to corresponding segments of the other chord.

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Circles — Interactive Study Guide

Circle Theorems Quick Reference

  1. Equal chords ⇔ equal central angles
  2. Perpendicular from centre bisects the chord
  3. Equal chords ⇔ equidistant from centre
  4. Central angle = 2 × inscribed angle (same arc)
  5. Angles in same segment are equal
  6. Angle in semicircle = 90°
  7. Opposite angles of cyclic quad = 180°

Problem-Solving Toolkit

For chord problems: Drop perpendicular from centre → bisects chord → use Pythagoras.
For angle problems: Identify if angle is at centre or circumference → apply the 2x rule.
For cyclic quad: Opposite angles add up to 180°.

Quick Self-Check

  1. Inscribed angle = 35°. Central angle for the same arc? (70°)
  2. Chord = 10 cm, distance from centre = 12 cm. Radius? (√(25+144) = √169 = 13 cm)
  3. ABCD is cyclic, ∠A = 95°. Find ∠C. (85°)

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