For the following A.P's write the first term and common difference:

3, 1, - 1, - 3, ...

In general, for an AP a₁, a₂, . . . .,a_n, we have

d = a_k+1 - a_k

where a_k+1 and a_k are the (k+1)^th and k^th terms respectively.

For the list of numbers: 3, 1, - 1, - 3, ...

a₂ – a₁ = 1 – 3 = -2

a₃ – a₂ =-1 – 1 = -1 - 1 = -2

a₄ – a₃ =-3 – (-1) = -3 + 1= -2

Here, the difference of any two consecutive terms in each case is --2. So, the given list is an AP whose first term a is 3 and common difference d is -2.