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

Triangle ABC is isosceles, in an isosceles triangle the angles at the base are equal. Angle C is the angle at the apex, so the angles at the base are equal:

angle A = angle B = (180 ° – 98 °): 2 = 82 °: 2 = 41 °.

Outside apex angle B = 180 ° – 41 ° = 139 °.

Answer: The outside angle at the vertex B is 139 °.

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