In an isosceles triangle ABC with base BC B = 40 degrees. Find the adjacent corner at vertex C.

Given:
isosceles triangle ABC,
ВС – base,
angle B = 40 degrees.
Find the degree measure of the adjacent angle at the vertex C -?
Solution:
Consider an isosceles triangle ABC. In it, the angles at the base are equal, that is, the angle B = angle C = 40 degrees. Knowing that the sum of the degree measures of the angles of a triangle is 180 degrees. We get:
angle A = 180 – 40 – 40;
angle A = 100 degrees.
The outer angle of a triangle is equal to the sum of the angles of the triangle that are not adjacent to it. Then the adjacent angle at the vertex C is equal to:
100 + 40 = 140 degrees.
Answer: 140 degrees.