12 An angle in a section is an angle comprehended of two right lines joyntly bounded in the base and in the periphery joyntly bounded 7 d iij.
Or thus: An angle in the section, is an angle comprehended under two right lines, having the same tearmes with the bases, and the termes with the circumference: H. As aoe, in the former example.
13 The angles in the same section are equall. 21. p iij.
Let the section be eauo, And in it the angles at a, & u: These are equall, because, by the [5 e], they are the halfes of the angle eyo, in the center: Or else they are equall, by the [7 e], because they insist upon the same periphery.
Here it is certaine that angles in a section are indeed angles in a periphery, and doe differ onely in base.
14 The angles in opposite sections are equall to two right angles. 22. p iij.
For here the opposite angles at a, and i, are equall to the three angles of the triangle eoi, which are equall to two right angles, by the [13 e vj]. For first i, is equall to it selfe: Then a, by parts is equall to the two other. For eai, is equall to eoi, and iao, to oei, by the [13 e]. Therefore the opposite angles are equall to two right angles.