\(\frac{a}{b}=\frac{9}{20}\Leftrightarrow\frac{a}{9}=\frac{b}{20}=k\Rightarrow a=9k;b=20k\)

a/b=9/20

cho kết quả r làm j lắm nữa

\(\frac{a}{b}=\frac{18}{40}\)

a) Đặt a/b = 7m / 15m ( m thuộc Z )

suy ra ƯCLN (7m;15m) = 6

(15;7) =1 suy ra m = 6

suy ra a/ b = 7.6/15.6 =42/90

b) 36/35 = 4/5

Đặt a/b = 4n / 5n ( n thuộc Z )

BCNN (a;b) = 300 suy ra BCNN(4n ; 5n ) = 300

(4;5) = 1 suy ra 4.5.n = 300 suy ra n=15 suy ra a/b = 4.15/5.15 = 60/75 

Ta có:

\(\frac{52}{a-20}=\frac{39}{b-15}=\frac{13}{c-5}\)

\(\Rightarrow\frac{a-20}{52}=\frac{b-15}{39}=\frac{c-5}{13}\)

\(=\frac{a}{52}-\frac{20}{52}=\frac{b}{39}-\frac{15}{39}=\frac{c}{13}-\frac{5}{13}\)

\(=\frac{a}{52}-\frac{5}{13}=\frac{b}{39}-\frac{5}{13}=\frac{c}{13}-\frac{5}{13}\)

\(\Rightarrow\frac{a}{52}=\frac{b}{39}=\frac{c}{13}\)

\(\Rightarrow\frac{a^2}{52^2}=\frac{b^2}{39^2}=\frac{c^2}{13^2}=\frac{bc}{39.13}=\frac{3}{3.13.13}=\frac{1}{13^2}\)

\(\Rightarrow\begin{cases}a^2=\frac{1}{13^2}.52^2=4^2\\b^2=\frac{1}{13^2}.39^2=3^2\\c^2=\frac{1}{13^2}.13^2=1^2\end{cases}\)\(\Rightarrow\begin{cases}a\in\left\{4;-4\right\}\\b\in\left\{3;-3\right\}\\c\in\left\{1;-1\right\}\end{cases}\)

Vậy giá trị (a;b;c) tương ứng thỏa mãn là: (4;3;1) ; (-4;-3;-1)