⁦The exterior angle, sides and angles, right triangles⁩

⁦Ukrainian Geometry 7⁩ (§§ ⁦10, 16, 17, 18) teaches these⁩: ⁦criteria for an isosceles triangle, the exterior angle, sides against angles, and congruence of right triangles⁩.

⁦An exterior angle of a triangle is the angle next to an interior angle (supplementary to it)⁩. ⁦It equals the sum of the two interior angles not next to it, so it is larger than each of them⁩.

⁦In a triangle the larger angle lies opposite the longer side, and the other way round⁩. ⁦So if two angles of a triangle are equal, it is isosceles⁩.

⁦Right triangles are congruent by⁩: ⁦two legs; a leg and an acute angle; the hypotenuse and an acute angle; the hypotenuse and a leg⁩.

⁦Drag the vertices and watch the numbers under the figure⁩.