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Inscribed and Central Angles in Circles

Definitions and theorems related to inscribed and central angles in circles are discussed using examples. See also interactive tutorial on central and inscribed angles.


1 - A central angle of a circle is an angle whose vertex is located at the center of the circle. Angle BOC in the figure below.

2 - An inscribed angle is an angle whose vertex is on a circle and whose sides each intersect the circle at another point. Angle CAB in the figure below.

definition of central and inscribed angles in a circle.


1 - An inscribed angle is half the measure of the central angle intercepting the same arc.

angle BAC = (1 / 2) angle BOC

angle BDC = (1 / 2) angle BOC

2 - Two or more inscribed angles intercepting the same arc are equal.

angle BAC = angle BDC

inscribed and central angles intercepting same arc.

Problem 1: In the figure below chord CA has a length of 12 cm. The circle of center O has a radius of 14 cm. Find an approximate value (2 decimal places) to the size of the inscribed angle CBA.

inscribed and central angles problem 1.

Solution to Problem 1:

  • We first calculate the central angle COA. Triangle COA is an isosceles triangle since length of CO = length of AO = radius = 14 cm. We use the cosine law to find cos (angle COA).

    CA 2 = CO 2 + AO 2 - 2 CO AO cos (angle COA)

  • Substitute CA, CO and AO by their numerical values and express cos(angle COA) as follows

    cos(angle COA) = [ 14 2 + 14 2 - 12 2 ] / [2 * 14 * 14 ]

    = 31 / 49

  • Size of angle COA is given by.

    Angle COA = arcos(31 / 49)

  • According to the theorem above, the size of angle CBA is equal to half the size of angle COA.

    Angle CBA = (1 / 2) arcos(31 / 49)

    = 25.38 degrees (approximated to 2 decimal places)

More geometry references
Geometry Tutorials, Problems and Interactive Applets.



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