We know that the sum of all the probabilities is $1$
Therefore, on integrating $\frac{1}{x^2}$ with limits a to $1$, the result should be $1$.
Hence, $\int_{a}^{1} \frac{1}{x^2} dx = 1$
$\left[-\frac{1}{x}\right]_a^1 = 1$
$-1 + \frac{1}{a} = 1$
Hence, a = 0.5