a)ta có \(\dfrac{a}{b}\)<\(\dfrac{c}{d}\)\(\Rightarrow\)\(\dfrac{a\times d}{b\times d}\)=\(\dfrac{c\times b}{d\times b}\)\(\Rightarrow\)a\(\times\)d=c\(\times\)d\(\Rightarrow\)ad=bc

b)theo câu a ta có \(\dfrac{a}{b}< \dfrac{c}{d}\Rightarrow ad=bc\)(1)

Thêm ab vào 2 vế của (1):ad+ab=bc+ab

a(b+d)<b(a+c)\(\Rightarrow\)\(\dfrac{a}{b}< \dfrac{a+c}{b+d}\)(2)

Thêm cd vào 2 vế của (1):ad+cd<bc+cd

d(a+c)<c(b+d)\(\Rightarrow\)\(\dfrac{a+c}{b+d}< \dfrac{c}{d}\)(3)

Từ(2)và(3)\(\Rightarrow\)\(\dfrac{a}{b}< \dfrac{a+c}{b+d}< \dfrac{c}{d}\)

2\(\dfrac{2}{3}\):0,03=1\(\dfrac{7}{9}\):x

\(\Rightarrow\)\(\dfrac{8}{3}\):0,03=\(\dfrac{16}{9}\):x

\(\Rightarrow\)\(\dfrac{800}{9}\)=\(\dfrac{16}{9}\):x

\(\Rightarrow\)\(\dfrac{16}{9}\):x=\(\dfrac{800}{9}\)

\(\Rightarrow\)x=\(\dfrac{16}{9}\):\(\dfrac{800}{9}\)

\(\Rightarrow\)x=\(\dfrac{1}{50}\)

Vậy x=\(\dfrac{1}{50}\)

cả 3 câu trên đều đúng. nhớ tick cho mình nhé

will go