Triangle ABC is isosceles, AC-base. Apex angle B is 120 degrees. Find the degree measures of the outer corners of the triangle.

A triangle is three points that do not lie on one straight line, connected by segments. In this case, the points are called the vertices of the triangle, and the segments are called its sides.

An isosceles triangle is a triangle in which the sides are equal.

In an isosceles triangle, the angles at the base are:

∠А = ∠С.

Since the sum of all the angles of the triangle is 180º, then:

∠А = ∠С = (180º – ∠В) / 2;

∠А = ∠С = (180º – 120º) / 2 = 60º / 2 = 30º.

Since the sum of the outer and inner angles of a triangle at one vertex is equal to 180º, then:

∠А1 = 180º – ∠А;

∠А1 = 180º – 30º = 150º;

∠В1 = 180º – ∠В;

∠В1 = 180º – 120º = 60º;

∠С1 = 180º – ∠С;

∠С1 = 180º – 30º = 150º.

Answer: the outer corners of the triangle are equal to ∠A1 = 150º; ∠В1 = 60º; ∠С1 = 150º.