A=3/2-5/6+/12-9/20+11/30-13/42+15/56-17/72+19/90

A=11/10

hok tốt nha

Cái này tôi thấy ko có tính nhanh đc 

bấm máy đi :))

Ra \(\frac{76}{405}\)

Áp dụng tích chéo

-5(y+5)=2(7-y)

-5y+(-25)=14-2y

-3y=39

y=39:(-3)=-13

Ủng hộ mk nha

y=-13

nhân chéo lên rồi giải

làm rồi mak

y=-13

Có \(P=\frac{1}{2}\times\frac{3}{4}\times\frac{5}{6}\times...\times\frac{399}{400}< \frac{2}{3}\times\frac{4}{5}\times...\times\frac{400}{401}\)

=> \(P^2< \frac{1}{2}\times\frac{2}{3}\times\frac{3}{4}\times...\times\frac{400}{401}=\frac{1}{401}< \frac{1}{400}=\frac{1}{20}\)

=> \(P< \frac{1}{20}\)(đpcm).

\(\frac{y+5}{7-y}=\frac{2}{-5}\Leftrightarrow-5.\left(y+5\right)=2.\left(7-y\right)\Rightarrow-5y-25=14-2y\Leftrightarrow-3y=39\Leftrightarrow y=-13\)

Ta co : \(\frac{y+5}{7-y}=\frac{2}{-5}\)

\(\Rightarrow-5.\left(y+5\right)=2.\left(7-y\right)\)

\(\Leftrightarrow-5y-25=14-2y\)

\(\Leftrightarrow-14-25=-2y+5y\)

\(\Leftrightarrow-39=3y\)

\(\Leftrightarrow y=-39:3\)

\(\Leftrightarrow y=-3\)

Vay y=-3

cảm ơn nhaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

a)121212/424242=2/7

1999999999/9999999995=1/5

Sorry bạn mik chỉ bt làm câu a thôi!

HT~

Câu b:

\(\frac{a}{b}:\frac{c}{d}=\frac{ad}{bc}=\frac{6}{5}\Leftrightarrow5ad=6bc\)

\(\frac{a}{b}-\frac{c}{d}=\frac{ad-bc}{bd}=\frac{1}{15}\Leftrightarrow5\left(ad-bc\right)=\frac{bd}{3}\)

\(\Rightarrow5ad-5bc=\frac{bd}{3}\)

Thay vào ta có:

\(\frac{a}{b}-\frac{c}{d}=\frac{a}{b}-\frac{1}{3}=\frac{1}{15}\Leftrightarrow\frac{a}{b}=-\frac{4}{15}\)