Читать книгу Geometry for Students and Parents - Sergey D Skudaev - Страница 6

Let́s start with triangles. Triangles have three sides and three angles.

There are three types of triangles. A triangle having three sides of equal length is called an equilateral triangle. A triangle having two sides of equal length is called an isosceles triangle. A triangle having three unequal sides is called a scalene triangle. See figure 10.

Figure 10. Different types of triangles.

The sum of the angles of a triangle equal 180 degrees. See figure 11.

Figure 11. The sum of the inner angles of a triangle.

BAC + ABC + ACB = 180

If a triangle has all three angles less than 90 degrees, it is called an acute triangle.

If a triangle has one angle equals 90 degrees, it is called a right triangle.

If a triangle has one angle that is greater than 90 degrees, it is called an obtuse triangle. See figure 12.

Figure 12. Different triangles.

When you want to refer to an angle, you denote it by 3 letters starting with a letter that denotes any angle side. For example, you can denote the angle ABC as CBA. Either way is correct, even though the first one may sound nicer.

An angle, which is formed by one side of a triangle and the continuation of the adjacent side of the same triangle, is equal to the sum of the internal, not adjacent, angles of the triangle. See figure 13.

Figure 13.