- Circle Theorems 3: Angle at the Centre Theorem Definitions An arc of a circle is a contiguous (i.e. no gaps) portion of the circumference. An arc which is half of a circle is called a semi-circle. An arc which is shorter than a semi-circle is called a minor arc. An arc which is greater than a semi-circle is called a major arc. Clearly, for every minor arc there is a corresponding major arc.

A segment of a circle is a figure bounded by an arc and its chord. If the arc is a minor arc then the segment is a minor segment. If the arc is a major arc then the segment is a major segment. Clearly, for every minor segment there is a corresponding major segment. A sector of a circle is a figure bounded by two radii and the included arc. If the arc is a minor arc then the sector is a minor sector. If the arc is a major arc then the sector is a major sector. Clearly, for every minor sector there is a corresponding major sector.

The word subtend means to hold up or support. For example, the minor arc BC subtends an angle

θ (∠BDC) at a point D on the major arc. ∠BDC at the point D.

We could also have said that the chord BC subtends Similarly, the chord or minor arc BC subtends

∠BAC at the centre A. The ∠BAC is

called a central angle and is sometimes measured by the length of the minor arc.

Copyright 2007, Hartley Hyde

Page 1 of 6

Twice the Angle

Prior Learning Theorem: The exterior angle of any triangle is equal to the sum of the interior opposite angles. Given: To Prove: any ABC with BC extended to D

m∠ACD = m∠BAC + m∠ABC

Construction: Draw CE // BA Proof: m∠ACE = m∠BAC = m∠ECD = m∠ABC = adding

α

(alternate ∠s: CE // BA)

β (corresponding ∠s: CE // BA) α+β

QED

⇒

m∠ACD = m∠BAC + m∠ABC =

Corollary:

The sum of the angles of any triangle is 180° ABC is any triangle Since

α + β + γ measure adjacent angles on the line BCD α + β + γ = 180°. ABC is also

It follows that

But the sum of the angles of the

α+β+γ

⇒

Purpose

The sum of the angles of any triangle is 180°

To demonstrate the Angle at the Centre of a Circle Theorem

from Book III of Euclid’s Elements

Angle at the Centre Theorem The angle which an arc of a circle subtends at the centre of a circle is double the angle which it subtends at any point on the remaining part of the circumference.

Copyright 2007, Hartley Hyde

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Twice the Angle

ClassPad Time Switch on your ClassPad and tap on the Geometry icon G. If your worksheet has data from a previous investigation, check if you need to save this before you clear it from the screen. Open the file ChordSt. From the Draw Menu tap on Point. Tap once somewhere on the major arc BC. This point will be automatically labelled D. From the View Menu tap on Select or get the Selection Arrow G from the Tool Bar. Highlight the line segments BD and CD.

From the Draw Menu tap on Attach Angle. Now you should find that ClassPad has measured ∠BDC for you. Tap on the radius BA and on the radius CA and both radii should be highlighted. From the Draw Menu tap on Attach Angle. Now you should find that ClassPad has measured ∠BAC for you.

Form a hypothesis relating m∠BDC and m∠BAC and write it in this space.

Checkpoint

Copyright 2007, Hartley Hyde

Page 3 of 6

Twice the Angle

Checking the hypothesis Try moving the point C. At each position of C around the circle you should find that m∠BDC = 2 × m∠BAC. Sometimes you get diagrams, like this one, where m∠BDC doesn’t look exactly double m∠BAC. From the View Menu tap on Select or get the Selection Arrow G from the Tool Bar. Highlight the radii AB and AC. Then tap on the arrow

p at the far right of the Tool

Bar. This takes you “around the corner” to the Measurement Bar. ClassPad has been programmed to anticipate that you wanted to know the size of the angle between the line segments you have highlighted. You can now see that any discrepancy is only because ClassPad is rounding the...