Proportion in a double triangle cut by a parallel
e b parallel to c d; ratio b d : d c equals a b : a c
Two hatched triangles are drawn between a pair of parallel lines and lettered for a proportion demonstration. Leonardo notes that e b is parallel to c d, and states that the ratio of b d to d c equals that of a b to a c. The pencil figures belong to the notebook's Euclidean study of the proportional division of triangles.
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'e b is parallel to c d'
A line e b is set parallel to the line c d across the paired triangles, establishing the condition for the proportion that follows.
Proportion b d : d c = a b : a c
Leonardo states that the ratio of segment b d to d c is the same as the ratio of a b to a c, dividing the double triangle proportionally.