Vanishing Concrete

A simple geometrical example will illustrate this. In order to determine and compare the areas of all rectilinear figures we split them up into triangles. Then the triangle itself is reduced to an expression totally different from its visible shape: half the product of the base and the altitude.

It's that "totally different from its visible shape" that is weird: true, a 

does not look like a

                                                

but what of it? A lot of what, according to KM...

In the same way the exchange values of commodities must be reduced to a common element, of which they represent a greater or lesser quantity. This common element cannot be a geometrical, physical, chemical or other natural property of commodities [because exchange values are quantities not qualities!!] clearly, the exchange relation of commodities is characterized precisely by its abstraction from their use-values.

Karl Marx "Capital Volume I" (1990) p127

So in abstracting the formula for the area of a triangle, I have extinguished all the triangles, made the triangleyness of the triangles disappear?