Q. 15 D4.1( 9 Votes )

# <span lang="EN-US

Since ABC = 120° and therefore the angle subtended by arc ABC in major arc will be:

APB = 180 – 120( By using Property of cyclic quadrilateral, where P is any point on circle)

APB = 60°

By the theorem:-

The angle formed at the centre of a circle by an arc, is double of the angle formed by the same arc at any point on circle.

AOC = 2 APB = 120°

In ΔAOB and ΔCOB,

OB = OB (given)

AB = BC (given)

ΔAOB ΔCOB by SSS congruency

ABO = CBO = 60°

Also, AOB = COB = 60° .

This shows ΔOAB is an equilateral triangle.

AB = OA = 5 cm

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