# Two circles of radii 5 cm and 3 cm intersect at two points and the distance between their centres is 4 cm. Find the length of the common chord. - Mathematics

Two circles of radii 5 cm and 3 cm intersect at two points and the distance between their centres is 4 cm. Find the length of the common chord.

#### Solution

Let the radius of the circle centered at O and O' be 5 cm and 3 cm respectively.

OA = OB = 5 cm

O'A = O'B = 3 cm

OO' will be the perpendicular bisector of chord AB.

∴ AC = CB

It is given that, OO' = 4 cm

Let OC be x. Therefore, O'C will be x − 4

In ΔOAC,

OA2 = AC2 + OC2

⇒ 52 = AC2 + x2

⇒ 25 − x2 = AC2 ... (1)

In ΔO'AC,

O'A2 = AC2 + O'C2

⇒ 32 = AC2 + (x − 4)2

⇒ 9 = AC2 + x2 + 16 − 8x

⇒ AC2 = − x2 − 7 + 8x ... (2)

From equations (1) and (2), we obtain

25 − x= − x2 − 7 + 8x

8x = 32

x = 4

Therefore, the common chord will pass through the centre of the smaller circle i.e., O' and hence, it will be the diameter of the smaller circle.

Length of the common chord AB = 2 O'A = (2 × 3) cm = 6 cm

Concept: Equal Chords and Their Distances from the Centre
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#### APPEARS IN

NCERT Class 9 Maths
Chapter 10 Circles
Exercise 10.4 | Q 1 | Page 179