Triangles on the Same Base, Between the Same Parallels
Two triangles share base AB. Their apexes C and D both sit on the same line, parallel to AB. Drag the points, then compare.
STEAM · Geometry Lab
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Why it works: Triangles ABC and ABD sit on the same base AB and their apexes lie on a line parallel to AB, so both triangles have the same base length and the same perpendicular height — and area = ½ × base × height gives equal areas, whatever their shape.