Free Geometry Basics Course •Part 1  Terms in Geometry •Part 2  Types of Angles •Part 3 •Postulates • Part 4 • Measuring Angles • Part 5 • Bisectors, Congruencies, Theorems • Part 6 • Transfersal Properties • Geometry Help • Mathematicians Review • Conic Sections • Pythagorean Theorem • •

Related Resources • Measurement Formulas • Recommended Resources • Area Calculator • Introduction to Polygons

An angle is defined as: where two rays or two line segments join at a common endpoint called the vertex. See part 1 for additional information.

Acute Angle

An acute angle measures less that 90° and can look something like these (angles between the grey rays):

Right Angle

A right angle measures exactly 90° and will look something like this:

Obtuse Angle

An obtuse angle measures more than 90° but less than 180° and will look something like these:

Straight Angle

A straight angle is 180°

Reflex Angle

An reflex angle is more than 180° but less than 360° and will look something like this:

Pairs of Angles:

Complementary Angles
Two angles adding up to 90° are called complementary angles.

ABD + DBC are Complementary

Supplementary Angles
Two angles adding up to 180° are called supplementary angles.

ABD + DBC are Supplementary

If you know the angle of ABD, you can easily determine what the DBC is by subtracting ABD from 180!

On to Part 3.

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