Ta có \(\frac{91}{1.4}+\frac{91}{4.7}+\frac{91}{7.10}+...+\frac{91}{88.91}=\frac{91}{3}.\left(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{88.91}\right)\)

\(=\frac{91}{3}.\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{88}-\frac{1}{91}\right)\)

\(=\frac{91}{3}.\left(1-\frac{1}{91}\right)=\frac{91}{3}.\frac{90}{91}=30\)

9/1.4+9/4.7+9/7.10+…+9/88.91=30

=91/3. (3/1.4+3/4.7+3/7.10+…+3/88.91)

=91/3. (1-1/4+1/4-1/7+1/7-1/10+…+1/88-1/91)

=91/3*90/91=30

#)Giải :

\(\frac{91}{1.4}+\frac{91}{4.7}+\frac{91}{7.11}+...+\frac{91}{88.91}\)

\(=\frac{91}{3}\left(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.11}+...+\frac{3}{88.91}\right)\)

\(=\frac{91}{3}\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+...+\frac{1}{88}-\frac{1}{91}\right)\)

\(=\frac{91}{3}\left(1-\frac{1}{91}\right)\)

\(=\frac{91}{3}.\frac{90}{91}=30\left(đpcm\right)\)

         #~Will~be~Pens~#

\(\frac{91}{1\cdot4}+\frac{91}{4\cdot7}+...+\frac{91}{88\cdot91}=\frac{1}{3}\left(91-\frac{91}{4}+\frac{91}{4}-\frac{91}{7}+...-\frac{91}{91}\right)\)

\(=\frac{1}{3}\left(91-1\right)=\frac{1}{3}\cdot90=30\)

Ta có :

\(\frac{91}{1.4}+\frac{91}{4.7}+\frac{91}{7.10}+...+\frac{91}{88.91}\)

\(=\frac{91}{3}.\left(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{88.91}\right)\)

\(=\frac{91}{3}.\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{88}-\frac{1}{91}\right)\)

\(=\frac{91}{3}.\left[1+\left(\frac{1}{4}-\frac{1}{4}\right)+\left(\frac{1}{7}-\frac{1}{7}\right)+...+\left(\frac{1}{88}-\frac{1}{88}\right)-\frac{1}{91}\right]\)

\(=\frac{91}{3}.\left[1-\frac{1}{91}\right]\)

\(=\frac{91}{3}.\frac{90}{91}\)

\(=\frac{8190}{273}=30\)

Vì \(30=30\)nên \(\frac{91}{1.4}+\frac{91}{4.7}+\frac{91}{7.10}+...+\frac{91}{88.91}=30\)

~ Hok tốt ~

\(A=\frac{1}{1\cdot4}+\frac{1}{4\cdot7}+\frac{1}{7\cdot10}+...+\frac{1}{91\cdot94}\)

\(A=\frac{1}{3}\left(\frac{3}{1\cdot4}+\frac{3}{4\cdot7}+\frac{3}{7\cdot10}+...+\frac{3}{91\cdot94}\right)\)

\(A=\frac{1}{3}\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{91}-\frac{1}{94}\right)\)

\(A=\frac{1}{3}\left(1-\frac{1}{94}\right)\)

\(A=\frac{1}{3}\cdot\frac{93}{94}=\frac{31}{94}\)

\(A=\frac{1}{1\cdot4}+\frac{1}{4\cdot7}+.....+\frac{1}{91\cdot94}\)

\(3A=\frac{3}{1\cdot4}+\frac{3}{4\cdot7}+......+\frac{3}{91\cdot94}\)

\(3A=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+.........+\frac{1}{91}-\frac{1}{94}\)

\(3A=1-\frac{1}{94}=\frac{93}{94}\)

\(A=\frac{93}{94}:3\)

3A=5(3/1*4+3/4*7+...+3/91*94)

3A=5(1-1/4+1/4-1/7+...+1/91-1/94)

3A=5(1-1/94)

3A=5.93/94

A=(5.93/94):3

A=155/94

Ta đặt

  \(A=\dfrac{5}{1\times4}+\dfrac{5}{4\times7}+...+\dfrac{5}{91\times94}\)

\(\dfrac{3}{5}A=\dfrac{3}{1\times4}+\dfrac{3}{4\times7}+....+\dfrac{3}{91\times94}\)

\(\dfrac{3}{5}A=1-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+...+\dfrac{1}{91}+\dfrac{1}{94}\)

\(\dfrac{3}{5}A=1-\dfrac{1}{94}=\dfrac{93}{94}\)

\(A=\dfrac{93}{94}\div\dfrac{3}{5}=\dfrac{155}{94}\)

A=1.4+1/4.7+1/7.10+...+1/91.94

=1/3.(3/1.4+3/4.7+3/7.10+...+3/91.94)

=1/3.(1-1/4+1/4-1/7+1/7-1/10+...+1/91-1/94)

=1/3.(1-1-94)

=1/3.(93/94)

=31/94

\(\frac{1}{1\cdot4}+\frac{1}{4\cdot7}+\frac{1}{7\cdot10}+...+\frac{1}{91\cdot94}\)

\(=\frac{1}{3}\left(\frac{3}{1\cdot4}+\frac{3}{4\cdot7}+\frac{3}{7\cdot10}+...+\frac{3}{91\cdot94}\right)\)

\(=\frac{1}{3}\left(1-\frac{1}{94}\right)\)

\(=\frac{1}{3}\cdot\frac{93}{94}\)

\(=\frac{31}{94}\)

\(A=\frac{1}{1\cdot4}+\frac{1}{4\cdot7}+.......+\frac{1}{91\cdot94}\)

\(3A=\frac{3}{1\cdot4}+\frac{3}{4\cdot7}+......+\frac{3}{91\cdot94}\)

\(3A=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+.......+\frac{1}{91}-\frac{1}{94}\)

\(3A=1-\frac{1}{94}=\frac{93}{94}\)

\(A=\frac{93}{94}:3\)