a: 20/30=2/3

30/45=2/3

=>20/30=30/45

b: -25/35=-5/7

-55/77=-5/7

=>-25/35=-55/77

\(\dfrac{20}{30}=\dfrac{20:10}{30:10}=\dfrac{2}{3}\)

\(\dfrac{30}{45}=\dfrac{30:15}{45:15}=\dfrac{2}{3}\)

Ta thấy\(\dfrac{2}{3}=\dfrac{2}{3}\Rightarrow\dfrac{20}{30}=\dfrac{30}{45}\)

phân số lào

lại lỗi rồi bn

Vù dụ ta có `(a)/(b)=(m)/(n)`

`=>a.n=b.m`

Để chứng tỏ 2 p/s bằng nhau , ta lấy tử của p/s này nhân mẫu của p/s kia . Mẫu củap/s này nhân tử của p/s kia

a) Ta có:

\(\dfrac{36}{84}=\dfrac{3}{7}\) ; \(\dfrac{42}{98}=\dfrac{3}{7}\)

\(\Rightarrow\dfrac{36}{84}=\dfrac{42}{98}\left(=\dfrac{3}{7}\right)\)

b) Ta có :

\(\dfrac{123}{237}=\dfrac{41}{79}\) ; \(\dfrac{123123}{237237}=\dfrac{41}{79}\)

\(\Rightarrow\dfrac{123}{237}=\dfrac{123123}{237237}\)

a) Ta rút gọn phân số:

\(\dfrac{36}{84}\)=\(\dfrac{36:12}{84:12}\)=\(\dfrac{3}{7}\)

\(\dfrac{42}{98}\)=\(\dfrac{42:14}{98:14}\)=\(\dfrac{3}{7}\)

Vì \(\dfrac{3}{7}\)=\(\dfrac{3}{7}\) nên \(\dfrac{36}{84}\)=\(\dfrac{42}{98}\)

Bài 5:

\(\dfrac{3}{4}=\dfrac{x}{20}=\dfrac{21}{y}\)

=>\(\left\{{}\begin{matrix}\dfrac{x}{20}=\dfrac{3}{4}\\\dfrac{21}{y}=\dfrac{3}{4}\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}x=20\cdot\dfrac{3}{4}=15\\y=21\cdot\dfrac{4}{3}=7\cdot4=28\end{matrix}\right.\)

Bài 4:

a: \(\dfrac{3}{5}=\dfrac{3\cdot9}{5\cdot9}=\dfrac{27}{45}\)

b: Đề sai rồi bạn

Bài 3:

a: \(21\cdot21=441\)

\(49\cdot9=441\)

=>\(21\cdot21=49\cdot9\)

=>\(\dfrac{21}{9}=\dfrac{49}{21}\)

b: \(\dfrac{-24}{34}=\dfrac{-24:2}{34:2}=\dfrac{-12}{17}\)

\(\dfrac{-60}{85}=\dfrac{-60:5}{85:5}=\dfrac{-12}{17}\)

Do đó: \(\dfrac{-24}{34}=\dfrac{-60}{85}\)

Bài 2:

\(15cm=\dfrac{3}{20}m\)

\(40mm=\left(\dfrac{40}{1000}\right)m=\dfrac{1}{25}m\)

Bài 1:

a: \(\left(-17\right):8=\dfrac{-17}{8}\)

b: \(\left(-8\right):\left(-9\right)=\dfrac{-8}{-9}\)

Ta có \(\left\{{}\begin{matrix}\left(3-2x\right)\left(3y-2x\right)\\\left(2x-3y\right)\left(2x-3\right)\end{matrix}\right.\left\{{}\begin{matrix}9y-6x-6xy+4x^2\\4x^2-6y-6xy+9y\end{matrix}\right.\\ =>\dfrac{3-2x}{2x-3y}=\dfrac{2x-3}{3y-2x}\)

123 237 = 123 . 1001 237 . 1001 = 123123 237237

36 84 = 36 : 12 84 : 12 = 3 7 42 98 = 42 : 14 98 : 14 = 3 7 D o   đ ó   36 84 = 42 98