Question

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Answer 1

$\frac{1}{1+a+ab}+\frac{1}{1+b+bc}+\frac{1}{1+c+ca}=\frac{1}{1+a+ab}+\frac{1}{1+b+\frac{1}{a}}+\frac{1}{1+\frac{1}{ab}+\frac{1}{b}}=$

$=\frac{1}{1+a+ab}+\frac{a}{1+a+ab}+\frac{ab}{1+a+ab}=\frac{1+a+ab}{1+a+ab}=1$