For a given $A.P.$,the first term is $-4$ and the common difference is $-5$. Then,the $12^{th}$ term of the $A.P.$ is $\ldots \ldots \ldots$.

The difference between two consecutive terms of an $A.P.$ is denoted by.........

Find the sum of the first $17$ terms of an $AP$ whose $4^{\text{th}}$ and $9^{\text{th}}$ terms are $-15$ and $-30$ respectively.

For an $A.P.$,the $7^{th}$ term is $-1$ and the $16^{th}$ term is $17$. Find the general term of the $A.P.$

Which of the following sequences is not an $A.P.$?

In which of the following situations,do the lists of numbers involved form an $AP$? Give reasons for your answers.
$(i)$ The fee charged from a student every month by a school for the whole session,when the monthly fee is $Rs. 400$.
$(ii)$ The fee charged every month by a school from Classes $I$ to $XII$,when the monthly fee for Class $I$ is $Rs. 250$,and it increases by $Rs. 50$ for the next higher class.
$(iii)$ The amount of money in the account of Varun at the end of every year when $Rs. 1000$ is deposited at simple interest of $10\%$ per annum.
$(iv)$ The number of bacteria in a certain food item after each second,when they double in every second.