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# Maths C2: Seems extremely simple but I don't get it!! watch

1. Currently Updating Question
2. (Original post by TwentyTen)
I know the principles, but I don't understand the proofs

The angle at the centre of a circle is twice the angle at the circumference.

Proof:

Let angle ACO=a and angle AOX=y

Angle CAO = angle ACO (isosceles triangle)

Therefore angle CAO = a

I get that bit, this is the bit that's confusing....

Hence y= 2a (exterior angle of triangle AOC)

Why?...How?
if AOX is the angle at the center, then it has to be 2x the angles at the circumference therefore if Y is the angle at the center and a stands for one of the angels at the circumference then 1/2y=a or y=2a
because the angels have equal angels they can be expressed as a but because the angle at the center is double you make it 2a=y

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Updated: February 12, 2010
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