In triangle ABC, the sides AC and BC are equal, the angle C is 98 degrees. Find the outside angle at vertex B.

From the condition it is known that in triangle ABC the sides АС and BC are equal, the angle С is equal to 98 °. In order to find the external angle at the vertex B, we first of all determine the shape of the triangle and find the degree measure of the angle B.

Since the sides are AC = BC, the triangle is isosceles, so the angles A and B are equal, as are the angles at the base of an isosceles triangle.

The angles of a triangle add up to 180 °.

2x + 98 = 180;

2x = 82;

x = 82: 2;

x = 41 ° angle A and B.

The sum of adjacent angles is 180 °.

Find the outer corner at vertex B.

180 ° – 41 ° = 139 °.