A simple equation to represent a whole subject? Sometimes the simplest of ideas can be the most powerful. Although a simplification we can consider:
Design and Technology = (Skills + Knowledge) x Values
With this simple theory our personal construct of the subject is the skills and knowledge that we consider to be important multiplied by the value given to them. For each of us involved in the subject domain, some skills and knowledge are more important than others. Perhaps traditional craft skills are ignored all together or, perhaps more likely, control technology. The values mulitiplier indicates that some of those selected skills are given more value than others.
OK, the theory needs a little work but presents an interesting starting point for discussion.