Ta có :\(\dfrac{a}{b}=\dfrac{c}{d}\rightarrow\dfrac{a}{b}.\dfrac{c}{d}=\dfrac{c}{d}.\dfrac{c}{d}=\dfrac{ac}{bd}\rightarrow\dfrac{ac}{bd}=\dfrac{c^2}{d^2}\)
\(\)\(\dfrac{a}{b}=\dfrac{c}{d}\rightarrow\dfrac{a}{b}.\dfrac{c}{d}=\dfrac{a}{b}.\dfrac{a}{b}=\dfrac{ab}{bd}\rightarrow\dfrac{a^2}{b^2}\)
\(\rightarrow\dfrac{ac}{bd}=\dfrac{c^2}{d^2}=\dfrac{a^2}{b^2}=\dfrac{c^2.a^2}{d^2.b^2}\)
\(\rightarrow\dfrac{ac}{bd}=\dfrac{a^2.c^2}{b^2.d^2}\left(đpcm\right)\)
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; B. 30; C. 
; D. 13.
