- Complementary Angles definition: Two angles whose measures add up to 90 degrees. If you know one angle, you can find the other by subtracting from 90 degrees.
Example: If one angle is 30 degrees, the complementary angle is 60 degrees (because 30+60=90 or 90-30=60)
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Supplementary Angles definition: Two angles whose measures add up to 180 degrees. If one angle is known, the other can be found by subtracting from 180 degrees.
Example: If one angle is 110 degrees, the supplementary angle is 70 degrees (because 110+70=180 or 180-110=70)
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Adjacent Angles definition: Two angles that share a common side and a vertex but do not overlap. The angles are next to each other; sometimes their sum can be 90 degrees (complementary) or 180 degrees (supplementary), but not always.
Example: Angles on the same straight line that meet at a point.
- Vertical (Opposite) Angles definition: When two lines intersect, the opposite (or vertical) angles formed are equal. Vertical angles are congruent (have the same measure).
Example: If two lines cross and one angle is 40 degrees, the opposite angle is also 40 degrees.
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Linear Pair definition: Two adjacent angles whose non-common sides form a straight line. Their measures add up to 180 degrees (they are supplementary).
Example: Two angles that lie on the same straight line, touching at a point.
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Corresponding Angles (when a transversal cuts two lines) definition: Angles that are in the same position relative to the two lines and the transversal. When the two lines are parallel, corresponding angles are congruent.
Example: If a transversal crosses two parallel lines, the top-left angle on the first line equals the top-left angle on the second line.
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Alternate Interior Angles (when a transversal cuts two lines) definition: Angles that are on opposite sides of the transversal and inside the two lines. If the lines are parallel, alternate interior angles are congruent.
Example: These pairs look like a “Z” shape between two lines.
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Alternate Exterior Angles (when a transversal cuts two lines) definition: Angles that are on opposite sides of the transversal and outside the two lines. If the lines are parallel, alternate exterior angles are congruent.
Example: These pairs look like a backward “Z” shape outside the two lines.
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Consecutive Interior Angles (Same-Side Interior Angles) definition: Angles on the same side of the transversal and inside the two lines. When the lines are parallel, these angles are supplementary (add to 180 degrees).
Example: These look like an “F” shape between the two lines.
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Right Angle definition: An angle exactly 90 degrees. Forms the basis of perpendicular lines; two lines that meet to form a right angle are perpendicular.
Acute and Obtuse Angles (not a relationship but good to know)
Acute Angle: Less than 90 degrees.
Obtuse Angle: Greater than 90 but less than 180 degrees.
Quick summary table:
Angle Relationship |
Angles Measures Property |
When Used |
Complementary |
Sum = 90° |
Finding unknown angles in right triangles, etc. |
Supplementary |
Sum = 180° |
Angles on a straight line, linear pairs |
Adjacent |
Share side and vertex |
Nearby angles |
Vertical |
Opposite angles equal |
Intersecting lines |
Linear Pair |
Adjacent & sum = 180° |
Angles on a straight line |
Corresponding |
Equal if lines are parallel |
Transversals cutting parallel lines |
Alternate Interior |
Equal if lines are parallel |
Transversals cutting parallel lines |
Alternate Exterior |
Equal if lines are parallel |
Transversals cutting parallel lines |
Consecutive Interior |
Sum = 180° if lines are parallel |
Transversals cutting parallel lines |
If you would like some visual aids to help, please let me know.