a. This looks to be arithmetic.....common difference between terms =2
And the first term = 2
So
an = 2 + 2(n - 1) which we can simplify as an = 2n
b. Also arithmetic .....common difference = 2 ....first term = 3
an = 3 + 2(n - 1) which we can simplify as an = 2n + 1
c. Arithmetic....common difference = -25 and the first term is 50
an = 50 - 25(n - 1) = 75 - 25n
4. Geometric.....common ratio = 1 / (1/3) = 3....first term = 1/3
an = (1/3)*3^(n - 1)