In an isosceles triangle ABC, the angle B is 80 degrees, AC is the base. Find the outside angle at the apex.

1. Proceeding from the fact that, in accordance with the properties of an isosceles triangle, the angles BAC and ABC at the base of the AU are equal, and the total value of its internal angles is 180 °, we calculate the magnitude of these angles:

Angle BAC = ACB = (180 ° – 80/2 = 100 °: 2 = 50 °.

2. We calculate the value of the external angle adjacent to the ACB angle:

180 ° – 50 ° = 130 °.

Answer: the outer angle of the triangle at the vertex C is 130 °.