# Find the angle A of a right-angled triangle ABC (angle C = 90 °) if angle B is 30 °, 70 °.

We know from the condition that a right-angled triangle ABC is given, and the angle C = 90 °. And it is also known that the angle B is equal to 1) 30 °; 2) 70 °.

In both cases, we need to find the degree measure of the third angle of triangle A.

To solve the problem, we apply the theorem on the sum of the angles of a triangle. The theorem says that the sum of the angles of a triangle is 180 °.

That is:

angle A + angle B + angle C = 180 °.

Let us express the angle A:

angle A = 180 ° – (angle B + angle C).

1) angle B = 30 °;

angle A = 180 ° – (30 ° + 90 °) = 180 ° – 120 ° = 60 °;

2) angle D = 70 °;

angle A = 180 ° – (70 ° + 90 °) = 180 ° – 160 ° = 20 °. One of the components of a person's success in our time is receiving modern high-quality education, mastering the knowledge, skills and abilities necessary for life in society. A person today needs to study almost all his life, mastering everything new and new, acquiring the necessary professional qualities.