Squaring the Triangle and Transforming Squares
Geometric constructions for the area of triangles and for turning rectangles and squares into one another.
A dense sheet of plane-geometry constructions. Leonardo shows how to find the area ("squaring") of any triangle by enclosing it between parallel lines so that a rectangle of the same base and half its height has an equal area, then works through transforming an oblong into a perfect square, combining two squares into one, and subtracting one square from another using the compass. The diagrams pair triangles inscribed in rectangles with squares and geometric-mean semicircles. On folio 239r he states a general rule that tapering figures of equal base between parallel lines are equal, and proves the construction of the square on a line a c.
On this page
Squaring any triangle between parallel lines
The area of any triangle is found by enclosing it between two parallel lines, with one side lying on one line and the opposite vertex touching the other. A rectangle built between the same lines on the triangle's base is then drawn, and half that rectangle equals the triangle.
Triangle equal to a rectangle of base and half-height
A triangle drawn inside a parallelogram is stated to equal a rectangle whose sides are the whole base of the triangle and half the height of the parallelogram. This restates the area rule in terms of the enclosing figure.
Turning an oblong into a perfect square
A short note headed by an incomplete phrase ("produced from the end of the smaller ...") sets the problem of converting a long rectangle into a perfect square. The adjacent semicircle construction supplies the geometric mean between the rectangle's sides.
Making one square out of two
The terse instruction "from two squares make a single one" heads the central semicircle diagram. The two squares are joined at a right angle so that the hypotenuse of the resulting right triangle becomes the side of the single equivalent square.
Subtracting one square from another (a, b, c)
Here a middle square is taken from the greater and the remainder equals the lesser square. With the compass Leonardo carries n to a, makes a c equal to n c (side of the greater square), and raises b c as the side of the right angle a b c, so that a b becomes the side of the smaller square.
Constructing the square on line a c
On folio 239r Leonardo proves that the product of a b by b d together with the square b c equals the greater square. He argues h equals g, and e and f are mutually equal, so that adding e and f to the squares h and i builds the greater square of line a c.
