1. What is an altitude of a triangle?

An altitude is the height of a triangle. It is a line from any vertex that meets the opposite side at right angles.

2. Is the longest side of a triangle always opposite the biggest angle?

Yes, and the SMALLEST angle is always opposite the SHORTEST side.

3. Can a triangle be both EQUILATERAL and RIGHT-ANGLED?

No, because each angle of an equilateral triangle is 60° so one of them cannot be a right angle!