As the sides of a Quadrangle are 4. Therefore the triangles which doe make the same foure-sided figure are but 2. The sides of a Quinquangle are 5, Therefore the triangles are 3, and so forth of the rest, as here thou seest. And
that indeed is the least: For even a triangle it selfe, may be cut into as many triangles as one please.
That both the inner and outter are equall to right angles, in every kinde of right line figure, it was manifest at the [4 e vj]. The inner in a Quadrangle, are equall to 4. In a Quinquangle, to 6: In an Hexangle, to 8; and so forth.
But the outter, in every right-lined figure, are equall to 4 right angles: as here may be demonstrated, by the [14 e v] and [13 e vj].
And
3. Homgeneall Triangulates are cut into an equall number of triangles, è 20 p vj.
For if they be Quadrangles, they be cut into two triangles: If Quinquangles, into 3. If Hexangles, into 4, and so forth.
4. Like triangulates are cut into triangles alike one to another and homologall to the whole è 20 p vj.
Or thus: Like Triangulates are divided into triangles like one unto another, and in porportion correspondent unto the whole: H.