Question

Answer 1

\(\dfrac{x}{3}=\dfrac{y}{4}=k\Rightarrow x=3k;y=4k\)Thay vào ta đc

\(D=\dfrac{12k-20k}{9k+16k}=\dfrac{-8k}{25k}=-\dfrac{8}{25}\)

Answer 2

\(\dfrac{x}{y}=\dfrac{3}{4}.\Rightarrow y=\dfrac{4}{3}x.\)

Thay vào biểu thức D ta có:

\(D=\dfrac{4x-5.\dfrac{4}{3}x}{3x+4.\dfrac{4}{3}x}=\dfrac{4x-\dfrac{20}{3}x}{3x+\dfrac{16}{3}x}=\dfrac{-\dfrac{8}{3}x}{\dfrac{25}{3}x}=\dfrac{-8}{25}.\)