If $\dfrac{a}{b} = \dfrac{c}{d}$, then

If $\dfrac{a}{b} = \dfrac{c}{d}$, then
  1. $ad = bc$
  2. $ac = bd$
  3. $a + b = c + d$
  4. $a - b = c - d$

Solution

Cross-multiplication.

Asked in: IMO

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