Đáp án:
B = $\frac{6}{5.7}$ + $\frac{6}{7.9}$ + ..... + $\frac{6}{57.59}$ + $\frac{6}{59.61}$
= 3.$\frac{2}{5.7}$ + 3.$\frac{2}{7.9}$ + ..... + 3.$\frac{2}{57.59}$ + 3.$\frac{2}{59.61}$
= 3. ($\frac{2}{5.7}$ + $\frac{2}{7.9}$ + ..... + $\frac{2}{57.59}$ + $\frac{2}{59.61}$)
= 3. ($\frac{1}{5}$ - $\frac{1}{7}$ + $\frac{1}{7}$ - $\frac{1}{9}$ + ..... + $\frac{1}{57}$ - $\frac{1}{59}$ + $\frac{1}{59}$ - $\frac{1}{61}$)
= 3. ($\frac{1}{5}$ - $\frac{1}{61}$)
= 3. $\frac{56}{305}$
= $\frac{168}{305}$
