Do you see why a triangle can contain at most one right angle?īecause this triangle contains an obtuse angle, it is called an obtuse triangle. Notice that the sum of the measures of these angles is 180°.īecause this triangle contains a right angle, it is called a right triangle. There are three classifications of a triangle according to the measures of its angles: acute, right, and obtuse.Įach of the angles in this triangle is acute, so this is called an acute triangle. The triangle is marked with congruence symbols to show which parts are congruent. Since the sum of the measures of the angles in a triangle is 180°, and all of the angles in an equilateral triangle are congruent, then each angle measures 180°÷ 3, or 60°. It is also equiangular, which means that all of its angles are also congruent. The third angle is called the vertex angle. In fact, according to the Isosceles Triangle Theorem, the base angles (those that are opposite the sides that are congruent) are also congruent. The two congruent sides are called the legs and the third side is called the base. That is, they all have different lengths. There are three classifications of a triangle according to the number of congruent sides it has: scalene, isosceles, and equilateral. To do so, remember that two figures are congruent if they have the same size and the same shape. Triangles may be classified by their sides or by their angles. Then we’ll explore 4-sided polygons, or quadrilaterals, and generalize to polygons with more than 4 sides. That is, they are the polygon with the least number of sides. Let’s begin with triangles, since they are the simplest polygons. Figure B is not a polygon, because it is not made of line segments figure C is not a polygon, because it is not closed and figure D is not a polygon, because two of the segments of which it is composed intersect three segments. Notice that each of the segments that compose the figure is joined with only two others. The point where each pair of segments intersects is called a vertex.Can you identify the figures below that are polygons and those that are not? So far, we’ve covered the basics of two-dimensional figures: points, lines, rays, planes, angles, and how to construct them.Ī polygon is a closed figure made by joining line segments, called sides, so that the line segments intersect exactly two other segments.In this lesson, we’ll begin to define, identify, and classify polygons. ⬅ Previous Lesson Workshop Index Next Lesson ➡
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