Take the B triangle and fold the vertex or the corner at the right angle over to the midpoint of the longer side of the triangle (here referred to as the base of the triangle).
This forms an isosceles trapezoid, called so because its two sides that are not parallel are equal .
The fold line turns out to be parallel to the base of the triangle and it also turns out to be half the length of the base of the triangle.
Open and Cut along the fold line. That should give you another isosceles right triangle , the medium-sized triangle of the tangram and a trapezoid.
Take the trapezoid . Notice that it has two acute angles and two obtuse angles.
Fold the trapezoid in half so that the two acute angles fold onto each other. Now you should have a trapezoid again.
Open and cut along the fold line. You should get two congruent trapezoids.