keep-flip-change

Let $a,b,c,d\\in\\mathbb{Z}-\\{0\\}$ . A mnemonic device for calculating

\\frac{a}{b}\\div\\frac{c}{d}

is KFC. This stands for ”keep-flip-change”. This means that you keep the first fraction as it is, flip the second fraction, and change the problem to a multiplication problem. Thus, the expression above becomes

\\frac{a}{b}\\cdot\\frac{d}{c}.

This yields that

\\frac{a}{b}\\div\\frac{c}{d}=\\frac{a}{b}\\cdot\\frac{d}{c}=\\frac{ad}{bc}.

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