The formula provided for the surface area of a triangular pyramid (SA = 1/2 ab + 3(1/2 lb)) is using the area of a triangle.
To elaborate, the term 1/2 ab relates to the area of the triangular base, where 'a' and 'b' are the base and height of the triangle respectively. The 3(1/2 lb) part likely refers to the lateral triangular faces of the pyramid, where 'l' is the slant height and 'b' is the base of the triangular faces.
So, the correct response is:
area of a triangle.