B1:
$\frac{1}{12}$ + $\frac{1}{20}$ + $\frac{1}{30}$ + ... + $\frac{1}{9702}$
= $\frac{1}{3.4}$ + $\frac{1}{4.5}$ + $\frac{1}{5.6}$ + ... + $\frac{1}{98.99}$
= $\frac{1}{3}$ - $\frac{1}{4}$ + $\frac{1}{4}$ - $\frac{1}{5}$ +...+ $\frac{1}{98}$ - $\frac{1}{99 }$
= $\frac{1}{3}$ - $\frac{1}{99 }$ = $\frac{32}{99 }$
B2:
a) 11.62+(-12).11+50.11 = 11. (62+-12+50)
= 11.100=1100
b) $\frac{5}{13}$ + $\frac{-5}{7}$ + $\frac{-20}{41}$ + $\frac{8}{13}$ + $\frac{-21}{24}$
= ($\frac{5}{13}$ + $\frac{8}{13}$) + $\frac{-5}{7}$ + $\frac{-20}{41}$ + $\frac{-7}{8}$
= 1+$\frac{-40}{56}$ + $\frac{-20}{41}$ + $\frac{-49}{56}$
=$\frac{56}{56}$ + $\frac{-40}{56}$ + $\frac{-49}{56}$+ $\frac{-20}{41}$
= $\frac{-33}{56}$ + $\frac{-20}{41}$
= $\frac{-1353}{2296}$ + $\frac{-1120}{2296}$= + $\frac{-2473}{41}$
