In an isosceles triangle ABC (BC – base), the apex angle is 20 degrees. Find the angle C.

By condition, an isosceles triangle ABC with base BC is given.
The value of the angle at the vertex of this triangle is also known.
Since BC is the base of triangle ABC, the known angle is A.
According to the rule, the sum of the values ​​of all angles in any triangle is 180 °.
Now let’s write for this triangle.
A + B + C = 180 °.
By condition, it is given that the angle A is equal to 55 degrees.
Let’s also recall the rule according to which the angles at the base are equal in an isosceles triangle.
That is, angle B is equal to angle C.
Now let’s find the corner C.
20 ° + B + C = 180 °.
Since B = C, we will write.
20 ° + 2C = 180 °.
2C = 180 ° – 20 °.
2C = 160 °.
C = 160 °: 2.
C = 80 °.
Answer: C = 80 °.