Which of the following triangles are impossible to draw?

a right obtuse triangle
an equilateral scalene triangle
an acute isosceles triangle
a right equilateral triangle
a right scalene triangle

* a right obtuse triangle

impossible, because if one angle is right (=90°), the other two must be acute (<90°)

* an equilateral scalene triangle

impossible, because these two types of triangles exclude each other - if a triangle is equilateral, it has three equal sides; if it's scalene, it has no equal sides

* an acute isosceles triangle

possible, for example a triangle with angles 40°, 70°, 70°

* a right equilateral triangle

impossible, each angle of an equilateral triangle measures 60°

* a right scalene triangle

possible, for example a triangle with sides 3, 4, 5

The answer is: a right obtuse triangle, an equilateral scalene triangle, a right equilateral triangle.