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Rule

Protractor Postulate

Consider a ray

O A .

The rays of the form

O B

can be matched one-to-one with the real numbers. Here,

B

is any point not on the line

O A .

Protractor, line OA and ray OB

The measure of

∠ A O B

is the absolute value of the difference of the real numbers matched to

O A

and

O B

on a protractor.

Measure of angle AOB

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