Câu d viết trên nhaa.

b) $\frac{1}{2×3}$ + $\frac{1}{3×4}$ + $\frac{1}{4×5}$ + $\frac{1}{5×6}$  

= $\frac{1}{2}$ - $\frac{1}{3}$ + $\frac{1}{3}$ - $\frac{1}{4}$ + $\frac{1}{4}$ - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{6}$ 

= $\frac{1}{2}$ - $\frac{1}{6}$

= $\frac{3}{6}$ - $\frac{1}{6}$ = $\frac{2}{6}$ = $\frac{1}{3}$ 

c) $\frac{1}{6}$ + $\frac{1}{12}$ + $\frac{1}{20}$ + $\frac{1}{30}$ + $\frac{1}{42}$ + $\frac{1}{56}$ 

= $\frac{1}{2×3}$ + $\frac{1}{3×4}$ + $\frac{1}{4×5}$ + $\frac{1}{5×6}$ + $\frac{1}{6×7}$ + $\frac{1}{7×8}$ 

= $\frac{1}{2}$ - $\frac{1}{3}$ + $\frac{1}{3}$ - $\frac{1}{4}$ + $\frac{1}{4}$ - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{6}$ + $\frac{1}{6}$ - $\frac{1}{7}$ + $\frac{1}{7}$ - $\frac{1}{8}$ 

= $\frac{1}{2}$ - $\frac{1}{8}$ 

= $\frac{4}{8}$ - $\frac{1}{8}$ = $\frac{3}{8}$ 

d) $\frac{2}{6}$ + $\frac{2}{12}$ + $\frac{2}{20}$ + $\frac{2}{30}$ + $\frac{2}{42}$ + $\frac{2}{56}$ 

= $\frac{2}{2×3}$ + $\frac{2}{3×4}$ + $\frac{2}{4×5}$ + $\frac{2}{5×6}$ + $\frac{2}{6×7}$ + $\frac{2}{7×8}$ 

= 2 × ($\frac{1}{2×3}$ + $\frac{1}{3×4}$ + $\frac{1}{4×5}$ + $\frac{1}{5×6}$ + $\frac{1}{6×7}$ + $\frac{1}{7×8}$)

= 2 × ($\frac{1}{2}$ - $\frac{1}{3}$ + $\frac{1}{3}$ - $\frac{1}{4}$ + $\frac{1}{4}$ - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{6}$ + $\frac{1}{6}$ - $\frac{1}{7}$ + $\frac{1}{7}$ - $\frac{1}{8}$)

= 2 × ($\frac{1}{2}$ - $\frac{1}{8}$)

= 2 × ($\frac{4}{8}$ - $\frac{1}{8}$)

= 2 × $\frac{3}{8}$ = $\frac{3}{4}$