Copy an Angle
Part of the Transformations & Constructions section of Coddy's Geometry journey. Lesson 54 of 61.
To copy an angle you copy a triangle. The Compass tool carries a radius: tap two points for its length, then the center, and it draws a circle of that size anywhere.
Draw a circle about B; it meets the arms at X and Y. With the Compass, draw the same circle about D, meeting the new ray at X'. Then carry the opening XY: a circle about X' with radius XY meets the first one at Y'. Now DX' = BX, DY' = BY and X'Y' = XY, so the triangles are congruent by SSS, and ∠X'DY' equals ∠XBY.
Challenge
Copy ∠ABC onto the ray DE: construct a line through D that makes the same angle with DE, then press Check. The Compass is unlocked.
Try it yourself
Construct it with a compass and straightedge
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Transformations & Constructions
7Drawing on Squared Paper
Lengths by PythagorasPerpendicular StepsSquares by AreaRight Triangles by AreaMidpoints on the GridRecap: Squared Paper10Perpendiculars and Parallels
Perpendicular From a PointPerpendicular at a PointParallel Through a PointTools You Have EarnedRecap: Parallels5Dilations
Scaling From a CenterShrinking a ShapeDilating About a PointNegative Scale FactorsRecap: Dilations8Compass and Straightedge
Circles Keep DistancesWhy the Sides MatchAngles You BuiltA 60° AngleRecap: Equilateral Triangle11Constructed Shapes
A Square on a SegmentThe Regular HexagonCopy an AngleA Tangent From a PointRecap: Constructed Shapes