Line MA is perpendicular to sides AB and AC of triangle ABC. Find the angle between straight lines MA and BC.

Answer: 90 degrees.
Explanation of the solution: Line MA is perpendicular to two straight lines AC and AB lying in the plane of triangle ABC, in addition, point A belongs to all three straight lines (MA, AC and AB), therefore, by the sign of perpendicularity of the straight line and plane MA, it is perpendicular to the plane ABC. And since the straight line MA is perpendicular to the plane ABC, and BC belongs to the plane ABC, then by definition of the straight line perpendicular to the plane, the straight lines MA and BC are perpendicular, that is, the angle between them is 90 degrees.