A. Use the distance formula to see if at least two sides are congruent. B. Use the slope formula to see if any sides are perpendicular. read more
Because if you define an isosceles triangle as having exactly 2 congruent sides, then you can’t apply the theorems you prove about isosceles triangles to equilateral triangles. You have to treat them like a special case to be proved separately. You have to reprove all isosceles triangle theorems again for equilateral triangles. Every. Single. One. read more