Tangram & basic concepts of geometry
Open the triangle, you should have a square. The fold line goes from one vertex or corner to the other corner or vertex. We call such a line a diagonal of the square.
Cut this square in half by cutting along the diagonal line . You should get two triangles.
Those two triangles have the same size and shape. We say that triangles like that are congruent. To be congruent is a way of relating two figures to each other.
Take the A triangle , fold it by the middle.
This forms an isosceles right triangle smaller than the one that we started with. The fold line is perpendicular to the longer side of the triangle. We call this line the altitude of the triangle.
Cut this triangle in half along the altitude. You should get two congruent triangles, the two larger triangles of the tangram.