Transmuting an obtuse triangle into a right triangle of equal area
Building any triangle on a given base between two parallels without changing its area
This geometry page states that any triangle can be transmuted into the shape of any given triangle without changing its quantity (area). Leonardo works the example of turning the obtuse triangle e f g into the right-angled triangle f g h, drawing a line a c parallel to the base f g and tangent to the apex. The diagram at the top shows two triangles enclosed between two horizontal parallels, sharing the lower base and reaching to the upper line.
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Transmuting an obtuse triangle into a right triangle of equal area
Any triangle can be transmuted into the figure of any given triangle without removing its quantity (area). Here the obtuse triangle e f g is turned into the right-angled triangle f g h by drawing the line a c equidistant from the base f g and tangent to the upper angle. On the same base one may then construct every sort of triangle.