How to Copy an Angle Using a Compass

Geometry Essentials For Dummies

The basic idea behind copying a given angle is to use your compass to sort of measure how wide the angle is open; then you create another angle with the same amount of opening.

Here's the proof diagram.

Copying an angle.

Refer to the figure as you work through these steps:

Draw a working line, l, with point B on it.
Open your compass to any radius r, and construct arc (A, r) intersecting the two sides of angle A at points S and T.
Construct arc (B, r) intersecting line l at some point V.
Construct arc (S, ST).
Construct arc (V, ST) intersecting arc (B, r) at point W.
Draw line BW and you're done.

About This Article

About the book author:

Mark Ryan has more than three decades’ experience as a calculus teacher and tutor. He has a gift for mathematics and a gift for explaining it in plain English. He tutors students in all junior high and high school math courses as well as math test prep, and he’s the founder of The Math Center on Chicago’s North Shore. Ryan is the author of Calculus For Dummies, Calculus Essentials For Dummies, Geometry For Dummies, and several other math books.