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   1. The right angle It measures 90° (degrees), the straight angle measures 180° and the full angle measures 360°. Thus, the degree is 1/90 of the right angle, 1/180 of the straight angle or 1/360 of the full angle.

   2. The protractor

It is an instrument used to measure angles. It is typically made of transparent plastic or glass with a fixed point (0) in the center of the base and an arc of 180º. It is used to measure a given angle and draw an angle of a specified number of degrees. The vertex of the angle should coincide with the point 0 of the protractor.

Example: the angle XOY measures 42º.

   Answer:

   3. Complementary Angles
   Two Angles are Complementary if they add up to 90 degrees (right angle). The angle 1 and 2 are complementary because they add up to 90°.

   Answer:

   4. Supplementary Angles
   Two Angles are supplementary if they add up to 180 degrees° (straight angle). In the picture, we see that the sum of the angles 3 and 4 make a straight angle of 180 degrees.

   Answers:

   5. Vertically opposite angles
   Those that have the same vertex and their sides are opposite rays. In the picture, the angles a and c, and b and d are opposite.
   These angles are equal, that means, a = c and b = d. We also note that a and b are supplementary and measure 180 º. c and b are also supplementary. As the common sum is b, we have a = c.
   Look the above picture and answer: