For each arithmetic progression (AP), find the first term and common difference: i) 3, 1, -1, -3, ... ii) -5, -1, 3, 7, ... iii) 1/3, 5/3, 9/3, 13/3, ... iv) 0.6, 1.7, 2.8, 3.9, ...

The first term is the first number in the sequence.

$$a = 3$$

The common difference $d$ is found by subtracting any term from its succeeding term.

$$d = a_2 - a_1 = 1 - 3 = -2$$ $$d = a_3 - a_2 = -1 - 1 = -2$$

The first term is $\boxed{a = 3}$ and the common difference is $\boxed{d = -2}$.

The first term is the first number in the sequence.

$$a = -5$$

$$d = a_2 - a_1 = -1 - (-5) = -1 + 5 = 4$$ $$d = a_3 - a_2 = 3 - (-1) = 3 + 1 = 4$$

The first term is $\boxed{a = -5}$ and the common difference is $\boxed{d = 4}$.

The first term is the first number in the sequence.

$$a = \frac{1}{3}$$

$$d = a_2 - a_1 = \frac{5}{3} - \frac{1}{3} = \frac{4}{3}$$ $$d = a_3 - a_2 = \frac{9}{3} - \frac{5}{3} = \frac{4}{3}$$

The first term is $\boxed{a = \frac{1}{3}}$ and the common difference is $\boxed{d = \frac{4}{3}}$.

The first term is the first number in the sequence.

$$a = 0.6$$

$$d = a_2 - a_1 = 1.7 - 0.6 = 1.1$$ $$d = a_3 - a_2 = 2.8 - 1.7 = 1.1$$

The first term is $\boxed{a = 0.6}$ and the common difference is $\boxed{d = 1.1}$.