Question

\(A=\frac{3}{6}+\frac{3}{12}+\frac{3}{20}+\frac{3}{30}+\frac{3}{42}+\frac{3}{56}\)

Answer 1

A=3/6+3/12+3/20+3/30+3/42+3/56

<=>1/3xA=1/6+1/12+1/20+1/30+1/42+1/56

<=>1/3xA=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8

<=>1/3xA=1/2-1/8=3/8

<=>A=3/8:1/3=9/8

Vậy A=9/8

Answer 2

Đặt 3 chung ra ta dc 1/6+1/12+...+1/56

A=1/2.3+1/4.3+...+1/87.8

A=1/2-1/3+1/3-1/4+...+1/7-1/8

Rút gọn ta dc A=1/2-1/8

tự quy đồng nhé