# Angles in Geometry

Definitions and properties of angles in geometry.

## An Angle in Geometry

An angle is the rotation required to superimpose one of two intersecting lines on the other.

## Right Angle

A right angle is an angle with measure equal to 90 degrees.

## Straight Angle

A straight angle is an angle with measure equal to 180 degrees.

## An Acute Angle

An acute angle is an angle with a measure between 0 and 90 degrees.

## An Obtuse Angle

An obtuse angle is an angle with a measure between 90 and 180 degrees.

## Complementary Angles

Two angles are complementary if the sum of their measures is equal to 90 degree.

Example: angles a and b with measures a = 19° and b = 71° are complementary since a + b = 90°

## Supplementary Angles

Two angles are supplementary if the sum of their measures is equal to 180 degrees.

Example: angles a and b with measures a = 122.1° and b = 57.9° are supplementary since a + b = 180°

## Exercises (see solutions below)

Given angles a, b, c, d, e and f with measures
a = 21°
b = 90.1°
c = 90°
d = 134.2°
e = 69°
f = 45.8°
1 ) Which of these angles are acute?
2 ) Which of these angles are obtuse?
3 ) Which pairs of angles are complementary?
4 ) Which pairs of angles are supplementary?

1 ) a, e and f
2 ) b and d.
3 ) a - e
4 ) d - f

## More References and Links to Geometry Tutorials

Questions on Angles with Solutions and Explanations
Geometry Tutorials and Problems
More To Explore