`d) 4/3.6 + 4/6.9 + 4/9.12 + 4/12.15`

`= 4/3  . (1/3 - 1/6 + 1/6 - 1/9 + 1/9 - 1/12 + 1/12 - 1/15)`

`= 4/3  . (1/3 - 1/15)`

`= 4/3  . 4/15`

`= 4/15`

`đ) 7/1.5 + 7/5.9 + 7/9.13 + 7/13.18 + 7/17.21`

`= 7/4  . (1 - 1/5 + 1/5 - 1/9 + 1/9 - 1/13 + 1/13 - 1/17 + 1/17 - 1/21)`

`= 7/4  . (1 - 1/21)`

`= 7/4  . 20/21`

`= 5/3`

Chú ý: Dấu `.` là dấu `×`

#flo2k9

cho mik câu trl hay nhất nha =))

d)

$\frac{4}{3.6}$ + $\frac{4}{6.9}$ + $\frac{4}{9.12}$ + $\frac{4}{12.15}$

=> $\frac{4.3}{3.6}$ + $\frac{4.3}{6.9}$ + $\frac{4.3}{9.12}$ + $\frac{4.3}{12.15}$

=> 4($\frac{3}{3.6}$ + $\frac{3}{6.9}$ + $\frac{3}{9.12}$ + $\frac{3}{12.15}$ )

=> 4( $\frac{1}{3}$ - $\frac{1}{6}$ + $\frac{1}{6}$ - $\frac{1}{9}$ + $\frac{1}{9}$ - $\frac{1}{12}$ + $\frac{1}{12}$ - $\frac{1}{15}$ )

=> 4($\frac{1}{3}$ - $\frac{1}{15}$) 

=> 4. $\frac{4}{15}$ 

=> $\frac{16}{15}$ 

đ) $\frac{7}{1.5}$+ $\frac{7}{5.9}$ + $\frac{7}{9.13}$ + $\frac{7}{13.17}$ + $\frac{7}{17.21}$ 

=> $\frac{7.4}{1.5}$+ $\frac{7.4}{5.9}$ + $\frac{7.4}{9.13}$ + $\frac{7.4}{13.17}$ + $\frac{7.4}{17.21}$ 

=> 7.($\frac{4}{1.5}$+ $\frac{4}{5.9}$ + $\frac{4}{9.13}$ + $\frac{4}{13.17}$ + $\frac{4}{17.21}$ )

=> 7. (1-$\frac{1}{5}$ +$\frac{1}{5}$ - $\frac{1}{9}$ +..-$\frac{1}{21}$ )

=> 7.( 1- $\frac{1}{21}$ )

=> 7. $\frac{20}{21}$ 

=> $\frac{20}{3}$