Finding the cube root of 81 by a rectangle rule
With a construction for transforming a parallelepiped into a cube
Leonardo sets himself the problem of finding the cube root of 81, laying down as a rule that one should build a rectangle as long as the whole number whose root is sought and always one unit wide. A caption records a further drawing for transforming a parallelepiped into a cube, annotated with the figures 4, 2 and 8. The lower margin carries a small construction in which a right-angled frame with a lettered top and a diagonal falls onto a baseline. The writing is oriented upside down relative to the notebook.
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The cube root of 81 by the rectangle rule
Leonardo declares he wants the cube root of 81. His rule: build a rectangle that is as long as the whole number whose root is sought, and always one unit wide.
Transforming a parallelepiped into a cube
A caption identifies a drawing meant for turning a parallelepiped into a cube, marked with the numbers 4, 2 and 8. The construction in the lower margin, with its lettered frame and diagonal, belongs to this problem of reshaping solids.