Classification of triangle according to number of congruent sides?

Answer 1

3 sides equal = equilateral (if and only if three angle equal and each = 60 degrees);

2 sides equal = isosceles (if and only if the two angles that are opposite these sides are equal. And that implies they must be acute);

0 sides equal = scalene (if and only if all angles also different).

"If and only if" is a mathematical device to say that each of the two statements implies the other, or that they are equivalent.