Đáp án:
A = $\dfrac{1}{5.6}$ + $\dfrac{1}{6.7}$ + $\dfrac{1}{7.8}$ + .... + $\dfrac{1}{24.25}$
= $\dfrac{1}{5}$ -$\dfrac{1}{6}$ + $\dfrac{1}{6}$ -$\dfrac{1}{7}$ + $\dfrac{1}{7}$ - $\dfrac{1}{8}$ +...+ $\dfrac{1}{24}$ - $\dfrac{1}{25}$
= $\dfrac{1}{5}$ - $\dfrac{1}{25}$
= $\dfrac{5}{25}$ - $\dfrac{1}{25}$
= $\dfrac{4}{25}$
B = $\dfrac{2}{3.5}$ + $\dfrac{3}{5.8}$ + $\dfrac{11}{8.9}$ + $\dfrac{13}{19.32}$ + $\dfrac{25}{32.57}$ + $\dfrac{30}{57.87}$
= $\dfrac{1}{3}$ - $\dfrac{1}{5}$ + $\dfrac{1}{5}$ - $\dfrac{1}{8}$ +$\dfrac{1}{8}$ - $\dfrac{1}{9}$ + $\dfrac{1}{19}$ -$\dfrac{1}{32}$ + $\dfrac{1}{32}$ - $\dfrac{1}{57}$ + $\dfrac{1}{57}$ - $\dfrac{1}{87}$
= $\dfrac{1}{3}$ - $\dfrac{1}{87}$
= $\dfrac{29}{87}$ - $\dfrac{1}{87}$
= $\dfrac{28}{87}$
Bài 5 sai đề :v
