*Bạn tham khảo nhé!!!!
Bài 3:
$D=\frac{1}{2*3} + \frac{1}{3*4} + \frac{1}{4*5} + ... + \frac{1}{19*20}$
$=\frac{1}{2} - \frac{1}{3} + \frac{1}{3} - \frac{1}{4} + ... + \frac{1}{19} - \frac{1}{20}$
$=\frac{1}{2} - \frac{1}{20}$
$=\frac{10}{20} - \frac{1}{20}$
$=\frac{9}{20}$
Bài 4:
a. x - $\frac{1}{2} = \frac{-2}{3}$
⇔x = $\frac{-2}{3} + \frac{1}{2}$
⇔x = $\frac{-1}{6}$
c. $\frac{-2}{5} + \frac{5}{6}x = \frac{-4}{15}$
⇔$\frac{5}{6}x = \frac{-4}{15} - \frac{-2}{5}$
⇔$\frac{5}{6}x = \frac{2}{15}$
⇔x = $\frac{2}{15} : \frac{5}{6}$
⇔x = $\frac{4}{15}$
d. $\frac{2}{3} + \frac{7}{4}:x=\frac{5}{6}$
⇔$\frac{7}{4}:x = \frac{5}{6} - \frac{2}{3}$
⇔$\frac{7}{4}:x = \frac{1}{6}$
⇔x = $\frac{7}{4} : \frac{1}{6}$
⇔x = $\frac{21}{2}$
e. $(x + \frac{5}{3})*(x - \frac{5}{4}) = 0$
⇔$\left \{ {{x+\frac{5}{3}=0} \atop {x-\frac{5}{4}=0}} \right.$ ⇔$\left \{ {{x=\frac{-5}{3}} \atop {x=\frac{5}{4}}} \right.$
