Question 1:   Find the formula of the n-th term of an arithmetic progression $\left(b_n\right)$ : $a+3, a+1, a-1, a-3,\dots$

Question 2:   In the arithmetic progression ($a_n$ ): $a_1=1;d=2.5$ . Is $13.5$ a term of this progression? If yes, find its number.

Question 3:   Find the initial term of the arithmetic sequence ($a_n$ ), if $b_5=11; b_{11}=-7$ .

Question 4:   What is the number of the first positive term of an arithmetic sequence ($a_n$ ): $-10.2;-9.5;-8.8;$…?

Question 5:   Find the first negative term of an arithmetic sequence ($a_n$ ): $7.2;6.6;6$ .