From similar figures to the Basic Proportionality Theorem, the similarity criteria, and measuring a tower by its shadow — all drawn out on the surveyor's blueprint table.
Same shape, not necessarily the same size. Two polygons are similar only when both conditions hold: corresponding angles equal, and corresponding sides proportional. Tap each pair to check.
Enlarging a photograph keeps every angle the same and stretches every side by the same ratio — the scale factor.
Drag D along AB and E along AC independently. When AD/DB = AE/EC, the theorem's converse guarantees DE is parallel to BC — watch the line snap to parallel and turn green.
You never need to check all six pairs of corresponding parts. Any one of these shortcuts is enough to prove two triangles similar. Tap a pair to check.
This is how heights of mountains and distant towers get measured — without ever touching them. Two right triangles, same angle of elevation, proportional sides.