$$\frac{ 1- { b }^{ -1 } + { b }^{ -2 } }{ 1-b+ { b }^{ 2 } }$$
$\frac{1}{b^{2}}$
$$\frac{b^{-2}\left(b^{2}-b+1\right)}{b^{2}-b+1}$$
$$b^{-2}$$
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