Lesson 2 of 20 · 4 moves
Equilateral Triangle
Build an equilateral triangle on the segment AB.
This is the very first proposition of Euclid’s Elements, written around 300 BCE. Euclid opens his whole book with the vesica.
A crossing of the two circles lies one radius from A and one radius from B, so joining it to both gives three equal sides.
The app snaps to every crossing, checks your construction exactly, and offers hints one move at a time.
Step by step
Circle centred on A through B.
Circle centred on B through A.
Join A to the upper crossing.
Join B to the same crossing.
What it shows
Euclid I.1: all three sides are radii of the same two circles, so they must be equal.
