2 problems here:
The first one(#74) does not look as if it has an error, the standard formula being a1r(n-1) to find the nth term.
The second problem (#75) completely eludes me, there are answers in the back of my book for both a & b that are all fractions, yet I cannot solve for any.(ie find a way to prove the sequence is either arithmetic or geometric)
Trying to divide to find r gives me a remainder, solving for x does not do much as sequences don't seem to involve zeros. There must be some other way to solve for x that I am failing to see.