CBSE Class 9 Mathematics Question 4 of 12

Circles — Question 5

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Question 5

Three girls Reshma, Salma and Mandip are playing a game by standing on a circle of radius 5 m drawn in a park. Reshma throws a ball to Salma, Salma to Mandip, Mandip to Reshma. If the distance between Reshma and Salma and between Salma and Mandip is 6 m each, what is the distance between Reshma and Mandip?

Answer

Let R, S and M be the position of Reshma, Salma and Mandip respectively.

From center O,

Draw OA, perpendicular to chord RS and OB, perpendicular to chord SM.

Three girls Reshma, Salma and Mandip are playing a game by standing on a circle of radius 5 m drawn in a park. Reshma throws a ball to Salma, Salma to Mandip, Mandip to Reshma. If the distance between Reshma and Salma and between Salma and Mandip is 6 m each, what is the distance between Reshma and Mandip? NCERT Class 9 Mathematics CBSE Solutions.

We know that,

Perpendicular from the center to the chord, bisects it.

⇒ AR = AS = RS2=62\dfrac{RS}{2} = \dfrac{6}{2} = 3 m

From figure,

⇒ OR = OS = OM = 5 m (Radii of circle)

In ∆ OAR,

By pythagoras theorem,

⇒ OR2 = OA2 + AR2

⇒ (5)2 = OA2 + (3)2

⇒ 25 = OA2 + 9

⇒ OA2 = 25 - 9

⇒ OA2 = 16

⇒ OA = 16\sqrt{16}

⇒ OA = 4 m

By formula,

Area of triangle = 12×\dfrac{1}{2} \times base x height

Area of ∆ ORS = 12\dfrac{1}{2} x OA x RS ........(1)

From figure,

In ∆ ORS for base OS, RC is the altitude.

Area of ∆ ORS = 12\dfrac{1}{2} x OS x RC ........(2)

From equation (1) and (2), we get :

12\dfrac{1}{2} x OS x RC = 12\dfrac{1}{2} x OA x RS

12\dfrac{1}{2} x RC x 5 = 12\dfrac{1}{2} x 4 x 6

⇒ RC x 5 = 24

⇒ RC = 245\dfrac{24}{5}

⇒ RC = 4.8 m

As OC is the perpendicular to the chord RM,

∴ OC bisects RM.

⇒ MC = RC

⇒ MC = RC = 4.8 m

⇒ RM = 2 RC = 2 x 4.8 = 9.6 m

Hence, the distance between Reshma and Mandip is 9.6 m.

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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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