Quadrilaterals are four-sided polygons. Trapezoids have exactly one pair of parallel lines.
Parallelograms have two pairs of parallel lines. There are three special kinds of parallelograms: A rectangle has four right angles. A rhombus has four congruent sides. A square has four right angles and four congruent sides.
Acute triangles have three acute angles. Obtuse triangles have one obtuse angle. Right triangles have one right angle.
Classifying by Sides Equilateral triangles have three congruent sides. Isosceles triangles have two congruent sides. Scalene triangles have zero congruent sides.
Finding Missing Angles The three angles of a triangle always add to 180°. Use a variable to stand for the missing angle and set an equation equal to 180. x + 49 + 47 = 180 x + 96 = 180 – 96 = -96 x = 84