As in the three figures aei, next before: This a parallelogramme hath common with a circle, as was manifest at the [28. e iiij].
And
12 A parallelogramme is the double of a triangle of a trinangle of equall base and heighth, 41. p j.
The comparison first in rate of inequality of a parallelogramme with a triangle, doth follow: As here thou seest in this diagramme. For a parallelogramme is cut into two equall triangles, by the antecedent. Therefore it is the double of the halfe.
And
13 A parallelogramme is equall to a triangle of equall heighth and double base unto it: è 42. p j.
As to aei, the triangle, the parallelogramme aoiu, is equall: because halfe of the parallelogramme is equall to the triangle: Therefore the halfes being equall, whole also shall be equall.
From whence one may