Final answer:
The given information provides the values of sin(a) and sin(b), but it lacks a specific question or context. Additional information is needed to provide a meaningful answer.
Step-by-step explanation:
The given information states that sin(a) = 4/5 and n/2 < a < n, and sin(b) = -2pi/5, where n < b < 3n/2. It seems that there might be some typos or missing information in the question, as it is not clear what we are supposed to do with this given information. Please provide more context or a specific question related to these expressions so that I can assist you further.