For the $A.P.$ $3, 8, 13, 18, \ldots$,$T_{n} = \ldots$

The ratio of the $11^{\text{th}}$ term to the $18^{\text{th}}$ term of an $AP$ is $2:3$. Find the ratio of the $5^{\text{th}}$ term to the $21^{\text{st}}$ term,and also the ratio of the sum of the first five terms to the sum of the first $21$ terms.

For a given $A.P.$,$S_{n} = 5n^{2} + 8n$. Hence $T_{n} = \ldots \ldots \ldots \ldots$

For the $A.P.$ $100, 98, 96, 94, \ldots$,find the value of $a$.

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

$1, 1, 2, 3, 5, 8, 13, 21, 34, \ldots$ is $\ldots \ldots \ldots \ldots . .$