Đáp án:

Bạn tham khảo nhé.

Giải thích các bước giải:

$A=\dfrac{-1}{20}+\dfrac{-1}{30}+\dfrac{-1}{42}+\dfrac{-1}{56}+\dfrac{-1}{72}+\dfrac{-1}{90}\\\Rightarrow-A=\dfrac1{20}+\dfrac1{30}+\dfrac1{42}+\dfrac1{56}+\dfrac1{72}+\dfrac1{90}\\\Rightarrow-A=\dfrac1{4.5}+\dfrac1{5.6}+\dfrac1{6.7}+\dfrac1{7.8}+\dfrac1{8.9}+\dfrac1{9.10}\\\Rightarrow-A=\dfrac14-\dfrac15+\dfrac15-\dfrac16+\dfrac16-\dfrac17+\dfrac17-\dfrac18+\dfrac18-\dfrac19+\dfrac19-\dfrac1{10}\\\Rightarrow-A=\dfrac14-\dfrac1{10}\\\Rightarrow-A=\dfrac3{20}\\\Rightarrow A=\dfrac{-3}{20}\\B=\dfrac5{2.1}+\dfrac4{1.11}+\dfrac3{11.2}+\dfrac1{2.15}+\dfrac{13}{15.4}\\\Rightarrow \dfrac B7=\dfrac5{2.7}+\dfrac4{7.11}+\dfrac3{11.14}+\dfrac1{14.15}+\dfrac{13}{15.28}\\\Rightarrow\dfrac B7=\dfrac12-\dfrac17+\dfrac17-\dfrac1{11}+\dfrac1{11}-\dfrac1{14}+\dfrac1{14}-\dfrac1{15}+\dfrac1{15}-\dfrac1{28}\\\Rightarrow\dfrac B7=\dfrac12-\dfrac1{28}\\\Rightarrow\dfrac B7=\dfrac{13}{28}\\\Rightarrow B=\dfrac{13}4$