In geometry, a central angle is an angle whose vertex is at the center of a circle. A central angle is formed by two radii (plural of radius) of a circle. The central angle is equal to the measure of the intercepted arc. An intercepted arc is a portion of the circumference of a circle encased by two line segments meeting at the center of the circle

Proof

Given Consider a circle C with center O , we consider an inscribed angle at B by the arc AC

To Prove

∡B= \(\frac{1}{2} \measuredangle AOC\)

Construction

Join the vertices A and C with center O. Also draw a line through B and O .

\( \overset{⏜}{AC} \cong \measuredangle AOC \) or \( \overset{⏜}{AC} \equiv \measuredangle AOC \)

Similarly, the measure of chord AC has equal influence to the measure of its central angle ∡AOC. So it is also written as

\( \overline{AC} \cong \measuredangle AOC \) or \( \overline{AC} \equiv \measuredangle AOC \)

Similarly, the measure of chord AC has equal influence to the measure of its arc AC. So it is also written as

\( \overline{AC} \cong \overset{⏜}{AC} \) or \( \overline{AC} \equiv \overset{⏜}{AC} \)

#### Inscribed angle theorem

An inscribed angle in a circle is formed by two chords that have a common end point on the circle. This common end point is the vertex of the angle. In the figure below, circle with center O has the inscribed angle ∡ABC. The other end points than the vertex, A and C define the intercepted arc AC of the circle.#### Theorem

The measure of an inscribed angle is half the measure of the intercepted arc.Proof

Given Consider a circle C with center O , we consider an inscribed angle at B by the arc AC

To Prove

∡B= \(\frac{1}{2} \measuredangle AOC\)

Construction

Join the vertices A and C with center O. Also draw a line through B and O .

SN | Statement | Reasons |

1 | ∆BCO is Isosceles | CO=BO |

2 | y=2x | Triangle exteriar angle theorem |

3 | b=2a | Triangle exteriar angle theorem |

4 | y+b=2(a+x) a+x= \( \frac{1}{2}(y+b)\) ∡B= \(\frac{1}{2} \measuredangle AOC\) |
Adding 2 and 3 |

#### Symbolic Notation

Due to the theorem given above, it is seen that, the measure of arc AC has equal influence to the measure of its central angle ∡AOC. So it is also written as\( \overset{⏜}{AC} \cong \measuredangle AOC \) or \( \overset{⏜}{AC} \equiv \measuredangle AOC \)

Similarly, the measure of chord AC has equal influence to the measure of its central angle ∡AOC. So it is also written as

\( \overline{AC} \cong \measuredangle AOC \) or \( \overline{AC} \equiv \measuredangle AOC \)

Similarly, the measure of chord AC has equal influence to the measure of its arc AC. So it is also written as

\( \overline{AC} \cong \overset{⏜}{AC} \) or \( \overline{AC} \equiv \overset{⏜}{AC} \)

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