In the triangle ABC, AC = BC. The external angle at vertex B is 121 °. Find the angle C.

∠CBA – is adjacent to the outer corner, therefore 180 ° = ∠CBA + 121 °
∠CBA = 180 ° -121 ° = 59 °
Since AC = BC, the triangle ABC is isosceles.
Hence ∠CBA = ∠CAB = 59 ° (by the property of an isosceles triangle)
By the theorem on the sum of the angles of a triangle:
180 ° = ∠CBA + ∠CAB + ∠C
180 ° = 59 ° + 59 ° + ∠C
∠C = 62 °
Answer: 62

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.