1. What is a vertex of a triangle?

A vertex is a corner. A triangle has three vertices.

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 ISOSCELES and RIGHT-ANGLED?

Yes, its angles would be 90o, 45o and 45o.

4. Is a triangle a polygon?

Yes. It is the polygon with the smallest number of sides.