In triangle ABC, angle A is 70 degrees, external angle at vertex B is 79. Find angle C.

Let us determine what the angle B will correspond to if from the condition of our problem we know for sure that the external angle at a given vertex is 79 °:

180 – 79 = 101.

Let us determine what the sum of the angles A and B will equal if from the condition of our problem we know for sure that the angle A is 70 °:

70 + 101 = 171.

Let us determine what, in this case, the angle C of the specified triangle will be equal to, because we know that all three angles must be 180 °:

180 – 171 = 9.

Answer: Under these initial conditions, the angle C = 9 °.