The right triangle has one 90 degree angle and two acute (< 90 degree) angles. Since the sum of the angles of a triangle is always 180 degrees... y + z = 90 degrees. The two sides of the triangle that are by the right angle are called the legs... and the side opposite of the right angle is called the hypotenuse.
They were called the Hollywood 10 and accused of being communist. I don't think you would find many people who can name them all.
HUAC was concerned that Communists were using the film industry to disseminate their ideas. Why did this tactic seem plausible? HUAC thought that by Communists using the film industry it would manipulate to spread their idea and influence. The tactic might have been plausible because it could have led people to believe that everything communism said was a lie so they could convert to communist.