Three numbers $a, b$ and $c$ will be in $\mathrm{A}.\mathrm{P}.$, if and only if $2b=$ .

Check whether the sequence $a-3, a-5, a-7,..\dots$ in $\mathrm{AP}$ or not?.

Check whether the sequence $\frac{1}{2}, \frac{1}{3}, \frac{1}{4}, \frac{1}{4},\dots \dots \dots$ is in $\mathrm{AP}$ or not?

Check whether the sequence $\frac{1}{2}, \frac{1}{3}, \frac{1}{4}, \frac{1}{5},\dots \dots \dots$ is in $\mathrm{AP}$ or not?

Check whether the sequence $9, 13, 17, 21, 25, \dots \dots$ is in $\mathrm{AP}$ or not?

Check whether the sequence $\frac{-1}{3}, 0, \frac{1}{3}, \frac{2}{3}, \dots \dots \dots .$ is in $\mathrm{AP}$ or not?

Check whether the following sequence $1,-1,1,-1,1,-1,.\dots$ is in $\mathrm{AP}$ or not?

If $a=5, d=6$, then find the corresponding $\mathrm{AP}$.