xxiv. An acute-angled triangle is one that has its three angles acute, as F.
xxv. An exterior angle of a triangle is one that is formed by any side and the continuation of another side.
Hence a triangle has six exterior angles; and also each exterior angle is the supplement of the adjacent interior angle.
xxvi. A rectilineal figure bounded by more than three right lines is usually called a polygon.
xxvii. A polygon is said to be convex when it has no re-entrant angle.
xxviii. A polygon of four sides is called a quadrilateral.
xxix. A quadrilateral whose four sides are equal is called a lozenge.