Bài 1 :
$\frac{19}{21}$ - $\frac{5}{7}$ = $\frac{19}{21}$ - $\frac{15}{21}$ = $\frac{4}{21}$
Bài 2 :
a, x : $\frac{1}{2}$ = $\frac{12}{20}$
x = $\frac{12}{20}$ x $\frac{1}{2}$
x = $\frac{12}{40}$ = $\frac{3}{10}$
b, $X$ x $\frac{1}{3}$ = 1 - $\frac{3}{4}$
$X$ x $\frac{1}{3}$ = $\frac{1}{4}$
X = $\frac{1}{4}$ : $\frac{1}{3}$
X = $\frac{3}{4}$