Chapter P - Preliminary Concepts - P.3 - Informal Geometry and Measurement - Exercises P.3 - Page 24: 29

Congruent

The triangle was exactly cut in half when the paper was folded up on itself. Since it was symmetrical about the seam where the paper is folded, both triangles on either side of the fold are the same. Thus, we can conclude that the two triangles are congruent.