Probability-4

A hunter's chace of shotinng an animal at a distance $'r'$ is $\frac{a^2}{r^2}$ (r greater than a).He fires when $r=2 a$ and if he misses he reloads & fire when $r=3 a ,4 a, ...$ .If he misses at a distance $'n a'$ ,the animal escapes. The odds against the hunters are $\frac{n+a}{n-b}$ .Then value of $a+b$

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