\[ \beta = \frac{\lambda D}{d} = \frac{5 \times 10^{-7} \times 2}{3 \times 10^{-4}} = 10 \times 10^{-3} \, \text{m} \]
For 3rd maxima:
\[ y_3 = 3\beta = 10 \times 10^{-3} \, \text{m} = 10 \, \text{mm} \]
Let $ P_n = \alpha^n + \beta^n $, $ n \in \mathbb{N} $. If $ P_{10} = 123,\ P_9 = 76,\ P_8 = 47 $ and $ P_1 = 1 $, then the quadratic equation having roots $ \alpha $ and $ \frac{1}{\beta} $ is: