22.4.3—Hf = E1 – E2

recall and use hf = E1 – E2.

Use —hf = e1 – e2 to connect the rule to the data and decision in the question.

This matters because —hf = e1 – e2 determines what can be inferred or chosen; begin with the stated conditions and keep the conclusion tied to the evidence.

Example: apply —hf = e1 – e2 to one small, clearly defined case, show the key step or comparison, and explain the result in words.

Boundary: —Hf = E1 – E2 is not a universal recommendation. Check the syllabus scope, assumptions, units and the limits of the evidence before generalising.