$P_n = 729 \cdot 81 \cdot 9 \cdot \ldots ;(\text{n terms})$
$= 3^6 \cdot 3^4 \cdot 3^2 \cdot \ldots$
$P_n = 3^{6+4+2+\cdots} = 3^{n(7-n)}$
$\Rightarrow \sum_{n=1}^{40} (P_n) = 3^6 + 3^5 + \cdots + (40\ \text{terms})$
$= 3^6\left[\dfrac{1-(\frac{1}{3})^{40}}{1-\frac{1}{3}}\right]$
$= \dfrac{3^6(3^{40}-1)}{2\cdot 3^{40}}$
$= \dfrac{3^{40}-1}{2\cdot 3^{33}}$
Comparing
$\alpha = 40,; \beta = 33$
$\Rightarrow \alpha+\beta = 73$