TA THẤY : \(\frac{13}{30}=\frac{39}{90},\frac{14}{30}=\frac{42}{90}\)

GIỮA \(\frac{39}{90},\frac{42}{90}\)CÓ \(\frac{40}{90},\frac{41}{90}\)

VÂY 2 PHÂN SÓ Ở GIỮA \(\frac{13}{30},\frac{14}{30}\)LÀ \(\frac{40}{90},\frac{41}{90}\)

 TA THẤY:\(\frac{22}{30}=\frac{66}{90},\frac{33}{30}=\frac{99}{90}\)

TỰ LÀM

\(\frac{20}{30}+\frac{2}{5}=\frac{20}{30}+\frac{12}{30}=\frac{32}{30}=\frac{16}{15}\)

\(=nhae\)

\(=\)Phúc Phạm Hồng

\(\frac{20}{30}+\frac{2}{5}=\frac{20}{30}+\frac{12}{30}=\frac{32}{30}=\frac{16}{15}\)

= 50 x (10 + 20 + 30 + 40)

= 50 x 50

= 2 500

A = 10 + 20 + 30 + 40 + 50 + 60 + 70 + 80 + 90

A = ( 10 + 90) + (20 + 80 ) + ( 30 + 70 ) + (40 + 60 ) + 50 

A = 100 + 100 + 100 +100 + 50

A = 400 + 50

A = 450

chúc bn hk tốt nhé !

A = 450 nhé

1/4*5+1/5*6+1/6*7+1/7*8=1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8

=1/4-1/8

1/8

1/20+1/30+1/42+1/56=1/8

\(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\)

\(=\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}\)

\(=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)

\(=\frac{1}{4}-\frac{1}{7}\)

\(=\frac{7}{28}-\frac{4}{28}\)

\(=\frac{3}{28}\)

    Chúc bạn học tốt

Ta có : 

\(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\)

\(=\)\(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}\)

\(=\)\(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)

\(=\)\(\frac{1}{4}-\frac{1}{7}\)

\(=\)\(\frac{3}{28}\)

Chúc bạn học tốt ~ 

Đáp án là B

đáp án B

; hay chia       

Sửa đề: \(A=\frac12+\frac56+\frac{11}{12}+\frac{19}{20}+\frac{29}{30}+\frac{41}{42}+\frac{55}{56}+\frac{71}{72}+\frac{89}{90}+\frac{109}{110}\)

\(=1-\frac12+1-\frac16+1-\frac{1}{12}+1-\frac{1}{20}+\cdots+1-\frac{1}{110}\)

\(=10-\left(\frac12+\frac16+\frac{1}{12}+\cdots+\frac{1}{110}\right)\)

\(=10-\left(1-\frac12+\frac12-\frac13+\frac13-\frac14+\cdots+\frac{1}{10}-\frac{1}{11}\right)\)

\(=10-\left(1-\frac{1}{11}\right)=10-\frac{10}{11}=\frac{110}{11}-\frac{10}{11}=\frac{100}{11}\)

Đm