![]() ![]() It will have three congruent altitudes, so no matter which direction you put that in a shipping box, it will fit. What about an equilateral triangle, with three congruent sides and three congruent angles, as with △ E Q U below? Not every triangle is as fussy as a scalene, obtuse triangle. What about the other two altitudes? If you insisted on using side G U ( ∠ D) for the altitude, you would need a box 9.37 c m tall, and if you rotated the triangle to use side D G ( ∠ U), your altitude there is 7.56 c m tall. You would naturally pick the altitude or height that allowed you to ship your triangle in the smallest rectangular carton, so you could stack a lot on a shelf.Īltitude for side U D ( ∠ G) is only 4.3 c m. Think of building and packing triangles again. How to Find the Altitude of a TriangleĮvery triangle has three altitudes. To get the altitude for ∠ D, you must extend the side G U far past the triangle and construct the altitude far to the right of the triangle. To get that altitude, you need to project a line from side D G out very far past the left of the triangle itself. The altitude from ∠ G drops down and is perpendicular to U D, but what about the altitude for ∠ U? We can construct three different altitudes, one from each vertex.įor △ G U D, no two sides are equal and one angle is greater than 90 °, so you know you have a scalene, obtuse (oblique) triangle. The height or altitude of a triangle depends on which base you use for a measurement. How big a rectangular box would you need? Your triangle has length, but what is its height? Imagine you ran a business making and sending out triangles, and each had to be put in a rectangular cardboard shipping carton. ![]() Obtuse triangles - One interior angle is obtuse, or greater than 90 °Īn altitude is a line drawn from a triangle's vertex down to the opposite base, so that the constructed line is perpendicular to the base.Acute triangles - All interior angles are acute, or each less than 90 °.Oblique triangles break down into two types: Right - One right angle ( 90 °) and two acute angles.Anglesīy their interior angles, triangles have other classifications: Most mathematicians agree that the classic equilateral triangle can also be considered an isosceles triangle, because an equilateral triangle has two congruent sides. Equilateral - Three sides are congruent.Scalene - No two sides are congruent (equal in length).By their sides, you can break them down like this: Sides You can classify triangles either by their sides or their angles. A triangle gets its name from its three interior angles. To find the altitude, we first need to know what kind of triangle we are dealing with. Use the Pythagorean Theorem to calculate altitudes for equilateral, isosceles, and right triangles.Construct altitudes for every type of triangle.Locate the three altitudes for every type of triangle.Recognize and name the different types of triangles based on their sides and angles.The apothem of a regular polygon is also the height of an isosceles triangle formed by the center and a side of the polygon, as shown in the figure below.įor the regular pentagon ABCDE above, the height of isosceles triangle BCG is an apothem of the polygon.After working your way through this lesson and video, you will be able to: The length of the base, called the hypotenuse of the triangle, is times the length of its leg. When the base angles of an isosceles triangle are 45°, the triangle is a special triangle called a 45°-45°-90° triangle. Base BC reflects onto itself when reflecting across the altitude. ![]() Leg AB reflects across altitude AD to leg AC. The altitude of an isosceles triangle is also a line of symmetry. So, ∠B≅∠C, since corresponding parts of congruent triangles are also congruent. Based on this, △ADB≅△ADC by the Side-Side-Side theorem for congruent triangles since BD ≅CD, AB ≅ AC, and AD ≅AD. Using the Pythagorean Theorem where l is the length of the legs. ABC can be divided into two congruent triangles by drawing line segment AD, which is also the height of triangle ABC. Refer to triangle ABC below.ĪB ≅AC so triangle ABC is isosceles. The base angles of an isosceles triangle are the same in measure. Using the Pythagorean Theorem, we can find that the base, legs, and height of an isosceles triangle have the following relationships: The height of an isosceles triangle is the perpendicular line segment drawn from base of the triangle to the opposing vertex. ![]() The angle opposite the base is called the vertex angle, and the angles opposite the legs are called base angles. Parts of an isosceles triangleįor an isosceles triangle with only two congruent sides, the congruent sides are called legs. DE≅DF≅EF, so △DEF is both an isosceles and an equilateral triangle. ![]()
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