In a triangle ABC AB = BC. The outside angle at the vertex B is 4. Find the angle C.

Let’s use one of the properties of the outer corner of the triangle and the property of the angles at the base of an isosceles triangle. The degree measure of the external angle B is equal to the sum of two internal angles that are not adjacent to it. In our case, we get:
∠ In ext. = ∠ А + ∠ С = 2 * ∠ С → ∠ С = ∠ В ext. / 2 = 4 ° / 2 = 2 °.
Answer: the degree measure of the angle C is 2 °.