1) F = $\frac{2}{1x5}$ + $\frac{2}{5x9}$ + ... + $\frac{2}{81x85}$
F = $\frac{1}{2}$ x ( $\frac{4}{1x5}$ + $\frac{4}{5x9}$ + ... + $\frac{4}{81x85}$
F = $\frac{1}{2}$ x ( 1 - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{9}$ + ... + $\frac{1}{81}$ - $\frac{1}{85}$ ))
F = $\frac{1}{2}$ x ( 1 - $\frac{1}{85}$ )
F = $\frac{1}{2}$ x $\frac{84}{85}$
F = $\frac{42}{85}$
Vậy F = $\frac{42}{85}$
2) G = $\frac{3}{4x9}$ + $\frac{3}{9x14}$+ .... + $\frac{3}{34x39}$
G = $\frac{3}{5}$ x ( $\frac{5}{4x9}$ + $\frac{5}{9x14}$ + ... + $\frac{5}{34x39}$
G = $\frac{3}{5}$ x ( $\frac{1}{4}$ - $\frac{1}{9}$ + $\frac{1}{9}$ - $\frac{1}{14}$ + ... + $\frac{1}{34}$ - $\frac{1}{39}$
G = $\frac{3}{5}$ x ( $\frac{1}{4}$ - $\frac{1}{39}$ )
G = $\frac{3}{5}$ x $\frac{35}{156}$
G = $\frac{7}{52}$
Vậy G = $\frac{7}{52}$
