For the $A.P.$ $-2, -4, -6, -8, \ldots,$ the common difference is............

For a given $A.P.$,the common difference is $5$ and its $15^{th}$ term is $72$. Find the first term of the $A.P.$ and its $50^{th}$ term.

The $A.P.$ with the first term $-3$ and the common difference $4$ is $ \ldots \ldots \ldots \ldots . $

The $4^{\text{th}}$ term from the end of the $AP: -11, -8, -5, \dots, 49$ is

For any given Arithmetic Progression $(A.P.)$,the common difference $d$ is equal to:

$A$ plastic toy consists of $14$ circular discs of different radii. The radius of the smallest disc is $1\, cm$, the radius of the next bigger disc is $2\, cm$, and so on, such that the radius of the biggest disc is $14\, cm$. Find the sum of the circumferences of all the discs in $cm$.