In right-angled triangle ABC, angle A is 40 degrees. Find the rest of the angles of this triangle.

A triangle is called rectangular, one of the angles of which is straight, that is, it is equal to 90 degrees.
Let triangle ABC have a right angle – angle C = 90 degrees.
Angle A is 40 degrees by convention.
From the theorem on the sum of the angles of a triangle it is known that the sum of all angles of any triangle is 180 degrees, then:
angle A + angle B + angle C = 180 degrees.
Let’s substitute in this expression the values ​​that we know:
40 degrees + angle B + 90 degrees = 180 degrees.
Let’s solve the resulting equation and find the value of the angle B:
angle B = 180 degrees – 90 degrees – 40 degrees;
angle B = 50 degrees.
Answer: angle B = 50 degrees, angle C = 90 degrees.