Triangles and Polygons Identifying triangles by their angles and sides. Determining if a figure is a polygon. Lesson 2-4 and 2-5 Tools A B acute triangle three acute angles obtuse triangle one obtuse angle isosceles triangle at least two conguent sides octagon 8 sided figure decagon 10 sided figure right triangle one right angle equilaterial triangle 3 congruent sides scalene triangle no congruent sides quadrilateral 4 sided figure pentagon 5 sided figure convex polygon a figure without gaps