\(B=\dfrac{49}{2\cdot9}+\dfrac{49}{9\cdot16}+\dfrac{49}{16\cdot23}+...+\dfrac{49}{65\cdot72}\)

\(B=\dfrac{49}{7}\cdot\left(\dfrac{1}{2}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{16}+\dfrac{1}{16}-\dfrac{1}{23}+...+\dfrac{1}{65}-\dfrac{1}{72}\right)\)

\(B=7\cdot\left(\dfrac{1}{2}-\dfrac{1}{72}\right)\)

\(B=7\cdot\left(\dfrac{36}{72}-\dfrac{1}{72}\right)\)

\(B=7\cdot\dfrac{35}{72}\)

\(B=\dfrac{\left(7\cdot35\right)}{72}\)

\(B=\dfrac{245}{72}\)

\(\dfrac{B}{7}=\dfrac{7}{2\cdot9}+\dfrac{7}{9\cdot16}+\dfrac{7}{16\cdot23}+...+\dfrac{49}{65\cdot72}\\ \dfrac{B}{7}=\dfrac{1}{2}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{16}+\dfrac{1}{16}-\dfrac{1}{23}+...+\dfrac{1}{65}-\dfrac{1}{72}\\ \dfrac{B}{7}=\dfrac{1}{2}-\dfrac{1}{72}\\ \dfrac{B}{7}=\dfrac{35}{72}\\ B=\dfrac{35}{72}\times7\\ B=\dfrac{245}{72} \)

Đề là ntn:

\(A=49\left(\dfrac{1}{2.9}+\dfrac{1}{9.16}+\dfrac{1}{16.23}+...+\dfrac{1}{65.72}\right):\dfrac{1}{3}-\dfrac{7}{36}\)

\(A=7\left(\dfrac{1}{2}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{16}+\dfrac{1}{16}-\dfrac{1}{23}+...+\dfrac{1}{65}-\dfrac{1}{72}\right):\dfrac{1}{3}-\dfrac{7}{36}\)

\(A=7\left(\dfrac{1}{2}-\dfrac{1}{72}\right):\dfrac{1}{3}-\dfrac{7}{36}\)

\(A=7.\dfrac{35}{72}:\dfrac{1}{3}-\dfrac{7}{36}\)

\(A=\dfrac{245}{72}:\dfrac{1}{3}-\dfrac{7}{36}\)

\(A=\dfrac{735}{72}-\dfrac{7}{36}=\dfrac{735}{72}-\dfrac{14}{36}=\dfrac{721}{36}\)

em mới học lớp 7 thôi ạ

A = 7 (7 / 2.9 + 7 / 9.16 + .......... + 7/65.72)

A=7( 1/2 - 1/9 +1/9 - 1/16 +......+1/65 - 1/72)

A= 7 ( 1/2 -1/72)

A= 7 . 35/72

A=245/72

\(A=\frac{7^2}{2.9}+\frac{7^2}{9.16}+\frac{7}{16.23}+.....+\frac{7^2}{65.72}\)

=\(7.\left(\frac{1}{2}-\frac{1}{9}+\frac{1}{9}-\frac{1}{16}+\frac{1}{16}-\frac{1}{23}+....+\frac{1}{65}-\frac{1}{72}\right)\)

=\(7.\left(\frac{1}{2}-\frac{1}{72}\right)\)

=\(7.\frac{35}{72}\)

=\(\frac{245}{72}\)

A=7x(\(\frac{1}{2}\)-\(\frac{1}{9}\)+\(\frac{1}{9}\)-\(\frac{1}{16}\)+.....+\(\frac{1}{65}\)-\(\frac{1}{72}\))

A=7x(\(\frac{1}{2}\)-\(\frac{1}{72}\))

A=7x\(\frac{35}{42}\)

A=\(\frac{245}{72}\)

\(C=\frac{7^2}{2.9}+\frac{7^2}{9.16}+\frac{7^2}{16.23}+...+\frac{7^2}{65.72}\)

\(C=\frac{7^2}{7}.\left(\frac{1}{2}-\frac{1}{9}+\frac{1}{9}-\frac{1}{16}+\frac{1}{16}-\frac{1}{23}+...+\frac{1}{65}-\frac{1}{72}\right)\)

\(C=7.\left(\frac{1}{2}-\frac{1}{72}\right)\)

\(C=7.\frac{35}{72}=\frac{245}{72}\)

Ta có : \(C=\frac{7^2}{2.9}+\frac{7^2}{9.16}+\frac{7^2}{16.23}+.....+\frac{7^2}{65.72}\)

\(\Rightarrow C=7\left(\frac{7}{2.9}+\frac{7}{9.16}+\frac{7}{16.23}+.....+\frac{7}{65.72}\right)\)

\(\Rightarrow C=7\left(\frac{1}{2}-\frac{1}{9}+\frac{1}{9}-\frac{1}{16}+.....+\frac{1}{65}-\frac{1}{72}\right)\)

\(\Rightarrow C=7\left(\frac{1}{2}-\frac{1}{72}\right)\)

\(\Rightarrow C=7.\frac{35}{72}=\frac{245}{72}\)

\(C=7\left(\frac{1}{2}-\frac{1}{9}+\frac{1}{9}-.....+\frac{1}{65}-\frac{1}{72}\right)..\)

\(C=7\left(\frac{1}{2}-\frac{1}{72}\right)..\)

\(C=?\)

\(C=\frac{7^2}{2.9}+\frac{7^2}{9.16}+\frac{7^2}{16.23}+....+\frac{7^2}{65.72}\)

\(C=\frac{7^2}{7}\cdot\left(\frac{1}{2}-\frac{1}{9}+\frac{1}{9}-\frac{1}{16}+\frac{1}{16}-\frac{1}{23}+....+\frac{1}{65}-\frac{1}{72}\right)\)

\(C=7\cdot\left(\frac{1}{2}-\frac{1}{72}\right)\)

\(C=7\cdot\frac{35}{72}=\frac{245}{72}\)

C = 49(1/2.9 ... 1/65.72)

C = 49(1/2 - 1/9 +....+ 1/65 - 1/72)

C = 49( 1/2 - 1/72)

C = bạn tự tính nhé

Có j không hiểu thì Ib mình

C=\(7\cdot\left(\frac{7}{2\cdot9}+\frac{7}{9\cdot16}+...+\frac{7}{65\cdot72}\right)\)

=\(7\cdot\left(\frac{1}{2}-\frac{1}{9}+\frac{1}{9}-\frac{1}{16}+...+\frac{1}{65}-\frac{1}{72}\right)\)

=\(7\cdot\left(\frac{1}{2}-\frac{1}{72}\right)\)

=\(\frac{245}{72}\)

Ta có: 

C = \(\frac{7^2}{2.9}+\frac{7^2}{9.16}+\frac{7^2}{16.23}+...+\frac{7^2}{65.72}\)

=> C = \(7.\left(\frac{7}{2.9}+\frac{7}{9.16}+\frac{7}{16.23}+...+\frac{7}{65.72}\right)\)

=> C = \(7.\left(\frac{1}{2}-\frac{1}{9}+\frac{1}{9}-\frac{1}{16}+\frac{1}{16}-\frac{1}{23}+...+\frac{1}{65}-\frac{1}{72}\right)\)

=> C = \(7.\left(\frac{1}{2}-\frac{1}{72}\right)\)

=> C = \(7.\frac{35}{72}=\frac{245}{72}\)

Nhìn kĩ là ra thôi :

 \(\frac{7^2}{2.9}+\frac{7^2}{9.16}+...+\frac{7^2}{65.72}\)

= \(7\left(\frac{7}{2.9}+\frac{7}{9.16}+...+\frac{7}{65.72}\right)\)

= \(7\left(\frac{1}{2}-\frac{1}{9}+\frac{1}{9}-\frac{1}{16}+...+\frac{1}{65}-\frac{1}{72}\right)\)

= \(7\left(\frac{1}{2}-\frac{1}{72}\right)\)

= \(7.\frac{35}{72}=3\frac{29}{72}\)