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Measuring Angles

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notes

Measuring Angles:

We use a protractor to measure the size of an angle in degrees.

Suppose you want to measure an angle ABC.

  1. Place the protractor so that the midpoint (M in the figure) of its straight edge lies on the vertex B of the angle.

  2. Adjust the protractor so that `bar"BC"` is along the straight-edge of the protractor.

  3. There are two ‘scales’ on the protractor: read that scale which has the 0° mark coinciding with the straight-edge (i.e. with ray `bar"BC"`).

  4. The mark shown by `bar"BA"` on the curved edge gives the degree measure of the angle. We write m ∠ABC = 40°, or simply ∠ABC = 40°.

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