Un = n*(n+1)/2
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an = a1 + d(n - 1)
There is no general answer. It depends on the information about the sequence that is already known.
The general (or nth) term is given by the equation t(n) = a + (n-1)d where a is the first term and d is the common difference between successive terms.
In a Geometric Sequence each term is found by multiplying the previous term by a common ratio except the first term and the general rule is ar^(n-1) whereas a is the first term, r is the common ratio and (n-1) is term number minus 1
You need an equation for the nth term of the sequence, or some other means of identifying the sequence. In general, they will be a+n, a+2n, a+3n and a+4n although some go for a, a+n, a+2n and a+3n.