Let the first term be $a$ and common difference $d$.
$ a + 19d = 30 $
$ a + 29d = 20 $
Subtract → $10d = -10 \Rightarrow d = -1$
Then $a = 49$
10th term = $a + 9d = 49 - 9 = 40$
The coefficients $1, 3, 5, 7, 9, \ldots$ form an arithmetic sequence,
and the powers of 2 form a geometric sequence.
Hence, the overall sequence is arithmetico-geometric.
In a sequence of $21$ terms, the first $11$ terms are in A.P. with common difference $2$ and the last $11$ terms are in G.P. with common ratio $2$. If the middle term of A.P. is equal to the middle term of G.P., then the middle term of the entire sequence is
Let $t_1$ be the first term. For the A.P.: $t_6=t_1+10$, $t_{11}=t_1+20$.
For the G.P. (terms $11$ to $21$ with ratio $2$): $t_{16}=t_{11}\cdot 2^5=32t_{11}$.
Given $t_6=t_{16}$: $t_1+10=32(t_1+20)\Rightarrow 31t_1=-630\Rightarrow t_1=-\dfrac{630}{31}$.
Middle term of entire sequence is $t_{11}=t_1+20=-\dfrac{630}{31}+20=-\dfrac{10}{31}$.
Let first term $a$, common difference $d$.
$a+(m-1)d=n \quad\text{and}\quad a+(n-1)d=m$.
Subtract: $(m-n)d=n-m \Rightarrow d=-1$.
Then $a=n+(m-1)=m+n-1$.
$t_{10}=a+9d=(m+n-1)+9(-1)=m+n-10$.
Let roots be $a-d,\;a,\;a+d$. Then
sum $=3a=9 \Rightarrow a=3$.
Sum of pairwise products $=3a^2-d^2=23 \Rightarrow 27-d^2=23 \Rightarrow d^2=4$.
Hence $d=\pm2$.