property: if a/b = c/d then ad = bc description: cross-multiply can someone explain this to me with proof? i don't understand why it works.
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you should be able to see this by now... $\dfrac a b = \dfrac c d$ $bd \dfrac a b = bd \dfrac c d$ $d a = b c$ $ad = bc$
it's sinking in
Or "step by step": From $\frac{a}{b}= \frac{c}{d}$ multiply both sides by b: $\frac{a}{b}b= \frac{c}{d}b$ so $a= \frac{bc}{d}$ multiply both sides by d: $ad= \frac{bc}{d}d$ so $ad= bc$.