“ If the measure of the sine line of the angle under consideration is equal to the measure of the sine line of one of the other angles they both must be acute, for it is impossible for them both to be obtuse, as we have shown; and it is likewise impossible for one to be acute and the other obtuse, for if this were so these angles would be like an acute and obtuse angle that share one sine line, the two of which already total 180 degrees without the inclusion of the third angle of the triangle, and all three angles of a triangle must total no more than 180 degrees (as was written in section 63) . ”

“ If one wishes to know the sine of an arc one should draw a radius line from one side of the of the arc to the circle's center and draw a line meeting this radius line forming a right angle. This line is equal to the arc's sine line. In the above example, lines CF and BE are drawn from arc BC, meeting at a right angle. ”

“ One should also know that when three angles are known the length of the sides cannot be deduced, for only their proportion to each other can be determined.
Section 110 In a right triangle there are only three necessary arrangements of quantities. Since the right angle is known only two other quantities need be known: another angle and side or two sides [or two angles which is not mentioned] . ”