Finding the area of a circle
Half the diameter (3 1/2) times half the circumference (11) gives 38 1/2
A pen circle divided by a vertical diameter, marked 22 at the top, 7 at the center and 38 1/2 at the bottom, with a faint horizontal segment across the upper part. The note asks for the area of the circle and gives the rule: multiply half the diameter, 3 1/2, by half the circumference, 11, to get 38 1/2. This value is the same great-circle area reused in the sphere problems on the neighbouring folios.
On this page
Area of a circle as half-diameter times half-circumference
The circle carries its diameter (7) and circumference (22), with the resulting area 38 1/2 written below. The construction expresses the area as the product of the radius and the semicircumference. It grounds the 38 1/2 that the facing pages take as the great-circle area of the ball.
Worked rule: 3 1/2 times 11 = 38 1/2
Leonardo takes half the diameter as 3 and 1/2 and half the circumference as 11. He multiplies the two to reach 38 and 1/2, which is the requested area of the circle. The step turns the 22-to-7 ratio into a concrete area figure.