For question 20, could you teach me what geometric and arithmetic progression is?
I don't think you need Geometric/Arithmetic Progression for that question. It is algebraic manipulation from what I can see, however, a very quick rundown:
Arithmetic progression is where the next term in your series differs from the previous by a common difference. All the terms are an equal distance apart. The formula for the n-th term of an arithmetic progression will take the form:
Where a is the first term, and d is the common difference.
A geometric progression is where the next term in your series differs from the previous by a common multiple. We multiply each term by some fixed common ratio, to get the next term. The formula for n-th term of an arithmetic progression will take the form:
Where a is the first term and r is the common ratio.
There is more detail over on the 2U threads if required!