\(\frac{1}{n\left(n+1\right)}=\frac{\left(n+1\right)-n}{n\left(n+1\right)}=\frac{1}{n}-\frac{1}{n+1}\)

\(P=\frac{1}{1999.2000}-\frac{1}{1998.1999}-...-\frac{1}{2.3}-\frac{1}{1.2}\)

\(=\frac{1}{1999}-\frac{1}{2000}-\frac{1}{1998}+\frac{1}{1999}-\frac{1}{1997}+\frac{1}{1998}-...-\frac{1}{2}+\frac{1}{3}-1+\frac{1}{2}\)

\(P=\frac{2}{1999}-\frac{1}{2000}-1\)

\(P+\frac{1997}{1999}=\frac{2}{1999}+\frac{1997}{1999}-\frac{1}{2000}-1=1-1-\frac{1}{2000}=-\frac{1}{2000}\)

ko biet

tiên sư mày

\(-\dfrac{4}{5}+\dfrac{2}{-10}=-\dfrac{4}{5}-\dfrac{2}{10}=-\dfrac{8}{10}-\dfrac{2}{19}=-\dfrac{10}{10}=-1\)

\(=\dfrac{-4}{5}+\dfrac{-1}{5}=\dfrac{-5}{5}=-1\)

-1

1

\(\dfrac{4}{7}\times\dfrac{7}{4}=1\) 

(rút gọn chéo)

1

\(=\dfrac{5}{8}\times\dfrac{6}{7}=\dfrac{5\times3}{4\times7}=\dfrac{15}{28}\)

= 15/28

\(\dfrac{15}{28}\)

\(\dfrac{3}{2}\)

\(\dfrac{8}{10}+\dfrac{7}{10}=\dfrac{15}{10}=\dfrac{3}{2}\)

\(=\dfrac{8}{10}+\dfrac{7}{10}=\dfrac{3}{2}\)

55/30=11/6

1 điểm là dg thi hả e

Sau 2 giờ chạy được 3/8+2/7=37/56(quãng đường)

\(=\dfrac{1}{12}\cdot5=\dfrac{5}{12}\)

1/60

\(\dfrac{1}{3}\times\dfrac{1}{4}\times\dfrac{1}{5}=\dfrac{1}{60}\)

1/3.1/4.1/5

=1/12.1/5

=1/60