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s1.5.4

a/b + b/a Trick

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When adding a fraction and its reciprocal, ab+ba\frac{a}{b} + \frac{b}{a}, you can use the formula:

ab+ba=2+(ab)2ab\frac{a}{b} + \frac{b}{a} = 2 + \frac{(a-b)^2}{ab}

For example, 57+75=2+(75)275=2+435=2435\frac{5}{7} + \frac{7}{5} = 2 + \frac{(7-5)^2}{7 \cdot 5} = 2 + \frac{4}{35} = 2\frac{4}{35}.

A popular variation involves a slight modification, like 1113+211\frac{11}{13} + \frac{2}{11}. This can be rewritten as (1113+1311)1111(\frac{11}{13} + \frac{13}{11}) - \frac{11}{11}, allowing you to use the trick and then adjust the result.

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