4.Tính nhanh (phân số) : A= 1/5.6 + 1/6.7 + 1/7.8 + .......... + 1/24.25 B= 2/3.5 + 3/5.8 + 11/8.19 + 13/19.32 + 25/32.57 + 30/57.87 5. Cho A =10/17 + 8/15 + 11/12 . Chứng tỏ A<2 GIÚP MIK VS MN

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