Triangles in two internally tangent circles
Two circle-and-triangle figures, with a reference to '10 of the fourth'
The folio bears two small figures, each a triangle constructed inside a circle, the pair of circles arranged so that they touch internally. A brief note, '10 of the fourth', points to a proposition - apparently the tenth of a fourth book - underlying the construction. The diagrams are lightly drawn in pencil with a few crossing interior lines.
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Two triangles in internally tangent circles
Two circles are drawn side by side, each enclosing a triangle, and set so that one circle touches the other from within. Faint lines cross the interiors, marking the triangles' sides and a few auxiliary segments. The pairing illustrates triangles built within internally tangent circles.
Reference: '10 of the fourth'
At the top right Leonardo notes '10 of the fourth', a shorthand citation of the tenth proposition of a fourth book. The reference anchors the drawn figures to an established geometric result. No further working is written out.