Reason with straight angles, full turns and vertically opposite angles.
Example 1
Two adjacent angles form a straight line. One is 55∘. What is the other angle, in degrees?
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Answer: 125
- Angles on a straight line total 180∘.
- The unknown angle is 180−55=125∘.
Example 2
Three angles surround a point without gaps or overlaps. Two measure 60∘ and 140∘. What is the third angle, in degrees?
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Answer: 160
- Angles making one full turn total 360∘.
- Subtract the given angles: 360−60−140=160∘.
Example 3
Three angles surround a point without gaps or overlaps. Two measure 90∘ and 120∘. What is the third angle, in degrees?
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Answer: 150
- Angles making one full turn total 360∘.
- Subtract the given angles: 360−90−120=150∘.