# How to Calculate Acute, Obtuse & Right Angles

Written by john woloch
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Angles are everywhere. The walls and ceiling of the room where you're sitting meet in angles; the sides of a stop sign form angles; the point of the pencil or pen in your hand also form an angle. Given the abundance of angles in the world, it's not surprising that geometricians have names for certain angles. The angles formed by walls and ceilings are right angles; the angles made by the sides of a stop sign are obtuse angles; and the point of a pencil or pen forms a right angle. Measuring, or calculating, acute, obtuse, and right angles is easy. An acute angle always has a measure greater than 0 and less than 90 degrees; an obtuse angle always has a measure greater than 90 and less than 180 degrees; a right angle is always 90 degrees.

Skill level:
Easy

• Protractor

## Instructions

1. 1

Place the angle you want to measure on the table in front of you so that its vertex is pointing to your left and one ray, or side, of the angle is parallel to the edge of the table in front of you. The vertex of an angle is the point where the two sides, or rays, of the angle meet.

2. 2

Place a protractor on top of the angle so that you can see the vertex of the angle through the small hole in the straight bottom edge of the protractor.

3. 3

Rotate the protractor so that straight rule along its bottom edge covers the ray of the angle that is parallel to the table edge. Be certain to keep the angle vertex visible through the hole in the protractor.

4. 4

Observe where the second ray of the angle intersects the curved top rule of the protractor.

5. 5

Note that if the ray intersects the curved rule to the right of the division on the curved rule marked "90," your angle is less than 90 degrees, so is an acute angle. Read the number on the lower curved rule where the ray intersects. For example, if you read, "70," your acute angle measures 70 degrees.

6. 6

Note that if the ray intersects the curved rule to the left of the division on the curved rule marked "90," your angle is greater than 90 degrees, so is an obtuse angle. Read the number on the lower curved rule where the ray intersects. For example, if you read, "170," your obtuse angle measures 170 degrees.

7. 7

Note that if the ray intersects the curved rule at the division on the curved rule marked "90," your angle is a right angle. The measurement of a right angle is always exactly 90 degrees.

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