For the $A.P.$ $-2, -4, -6, \ldots,$ the common difference $d = \ldots$

For the $A.P.$ formed by positive multiples of $4$,$d = \ldots \ldots \ldots \ldots .$

For a given finite $A.P.$,$a=1, l=10$ and $n=10$. Then,$S_{10} = ........$

Determine whether the following sequence is an $A.P.$ or not. (Assume that the pattern continues.) If it is an $A.P.$,find its $n^{th}$ term: $111, 107, 103, 99, \ldots$

The $10^{th}$ and $20^{th}$ terms of an $A.P.$ are $73$ and $143$ respectively. Find the first term,the common difference,and the $30^{th}$ term of the $A.P.$

Let $S_n$,$S_{2n}$,and $S_{3n}$ be the sums of $n$,$2n$,and $3n$ terms of an Arithmetic Progression $(AP)$,respectively. Prove that $S_{3n} = 3(S_{2n} - S_n)$.