Challenging +1.8 Part (a) is a straightforward proof using compound angle formulae, accessible to strong A-level students. Part (b) requires recognizing the Pythagorean identity to simplify sec²θ, then applying the compound angle formula for tan(A-B) in reverse, followed by solving a cotangent equation—this multi-step problem-solving with reciprocal functions and non-standard angles elevates it significantly above typical A-level questions but remains within reach of well-prepared students.