Theorem
The angle subtended by an arc at the centre is double the
angle subtended by it at any point on the remaining part of the
circle
The three cases
We have to consider three cases
I. When arc PQ is minor arc
II. When arc PQ is a semicircle
III. When arc PQ is major arc
Case-1
Arc PQ is minor arc
A
O
B
P
Q
Case-2

Arc PQ is Semicircle
A
O
P
B
Q
Case-3

A
Arc PQ is major arc
P
Q
O
B
Proof
Given :Arc PQ of a circle subtending ∟POQ at
the centre o and ∟PAQ at a point A on the
remaining part of the circle.
To prove: ∟POQ = 2 ∟PAQ
 Construction: join AO and extend it to a point
B
 Proof : In all the cases ∟BOQ = ∟OAQ
+∟AQO(exterior angle is equal to the sum of
the interior opposite angles)
Angles in a segment

Angles in the same segment of the circle
are equal(Theorem 10.9)
Theorem 10.10

A line segment joining two points subtends equal
angles at two other points lying on the same side of
the line containing the line segment, the four points
lie on a circle