Given a triangle ABC, angle A = 70 degrees, angle C = 40 degrees. Find angle B.

Let us determine what the sum of the known angles of the indicated triangle will equal, if from the condition of the problem we know exactly that angle A is equal to 70 °, while angle C is 40 °:

40 + 70 = 110.

Let us determine what, in this case, the angle B of the indicated triangle will be equal to, because we know from the school geometry course that the sum of the angles in such a geometric figure is 180 °:

180 – 110 = 70.

Answer: With such initial data, the angle B will be equal to 70 °, that is, it will be equal to the angle A.